WHITE MOUNTAINS INSURANCE GROUP LTD

Overview

White Mountains Insurance Group Ltd. is a diversified insurance and financial-services holding company. Its businesses include property and casualty insurance and reinsurance, municipal-bond guarantee reinsurance, capital solutions for asset- and wealth-management firms, specialty insurance distribution, and investment-related activities.[1] Its Ark business underwrites specialty property, casualty, marine and energy, and accident and health risks, while HG Global/HG Re focuses on municipal-bond guarantee reinsurance.[2][1]

The group’s operations include Lloyd’s and UK-based underwriting, Bermuda insurance and reinsurance activity, and corporate offices in Bermuda and the United States.[3]+2 White Mountains describes its approach as building long-term partnerships with insurance and related financial-services businesses.[6]

As a scale indicator, Ark reported $2.557 billion in gross written premiums for 2025; this is an operating-business premium measure, not consolidated group revenue.[7] The latest full-year figures located are for 2025, while the company also reported for the quarter ended June 30, 2026.[8][9] [1]+8

Products & services

White Mountains’ specialty insurance and reinsurance offerings include Ark’s niche property, specialty, marine and energy, casualty, and accident and health coverages. Property policies cover physical loss or damage, business interruption, and natural and non-natural catastrophe perils; accident and health offerings include personal accident, sickness, disability, travel, short-term life, health and medical risks, on both insurance and reinsurance bases. WM Outrigger provides collateralized reinsurance protection for Ark’s Bermuda global property-catastrophe excess-of-loss portfolio. In 2025, Ark reported $2.557 billion of gross written premiums, $1.613 billion of net earned premiums and $265 million of pretax income.[1]+4

HG Global’s HG Re reinsures Build America Mutual under a first-loss treaty, providing protection of up to 15% of the par amount outstanding for each policy assumed. In 2025, HG Global reported $61 million of gross written premiums, $3.170 billion in par value of policies assumed and $45 million of pretax income. Distinguished Programs provides specialty property-and-casualty program services as a managing general agent and program administrator, including product development, marketing, underwriting and policy issuance for carrier partners; its programs focus on areas including real estate and hospitality.[12]+2

Until its December 5, 2025 sale, Bamboo distributed homeowners’ insurance and related residential-property products in California and Texas, including admitted and non-admitted offerings and access to ancillary flood and earthquake products through a retail agency. Its MGA services included product development, marketing, underwriting, policy issuance and claims oversight. Kudu provides capital solutions to asset- and wealth-management firms rather than insurance policies, investing in firms across areas including real estate, real assets, wealth management, hedge funds, private equity and alternative credit. The 2025 reporting describes results by business but does not provide product-line percentages of company performance.[14]+2 [1]+10

Geographic presence

White Mountains Insurance Group is a Bermuda-exempted company headquartered at 26 Reid Street in Hamilton, Bermuda, with its principal executive office in Hanover, New Hampshire. Its insurance and reinsurance businesses Ark, WM Outrigger Re and HG Global are headquartered in Hamilton; Ark also has underwriting offices in London and Hamilton. Kudu, Distinguished and WTM Partners are headquartered in New York City.[17][18]

Ark operates in the U.K. Lloyd’s market and underwrites property insurance worldwide, with a focus on U.S. excess-and-surplus lines and large international accounts. Its property reinsurance is also worldwide, with particular emphasis on U.S. and European risks; its largest natural-catastrophe exposure zones are U.S. windstorm and earthquake.[19]+2 Ark sources Bermuda reinsurance through intermediaries representing ceding companies and insurance through brokers based in Bermuda and London.[3] WM Outrigger Re provides collateralized reinsurance for Ark’s Bermuda global property-catastrophe portfolio.[2]

The available company reporting does not provide revenue, premium or employee figures broken down by country or region.[17] [17]+6

Business segments

For the year ended December 31, 2025, White Mountains reported business areas including Ark/WM Outrigger, HG Global, Kudu, Distinguished, Bamboo, and Other Operations. Bamboo was sold on December 5, 2025 and ceased to be a reportable segment thereafter. The approximate revenue shares below use consolidated 2025 revenue of $3,735 million unless another measure is specified; they are not directly comparable across business types.[1]+2

- **Ark/WM Outrigger** — Specialty property and casualty insurance and reinsurance, including property, specialty, marine and energy, casualty, accident and health, and collateralized catastrophe reinsurance. Its $2,557 million of gross written premiums (GWP) was about 97.7% of combined Ark/WM Outrigger and HG Global GWP; its revenue of $1,943.8 million was about 52.1% of consolidated revenue.[2][23] - **HG Global** — Municipal bond guarantee reinsurance; HG Re provides first-loss protection of up to 15% of par for policies assumed from BAM. Its $61 million of GWP was about 2.3% of combined Ark/WM Outrigger and HG Global GWP; its revenue of $73.6 million was about 2.0% of consolidated revenue.[24][14] - **Kudu** — Capital solutions and strategic advice for boutique asset and wealth managers, including ownership transfers, buyouts, growth financing and partner liquidity. Revenue of $183.4 million was about 4.9% of consolidated revenue; it includes investment-related revenue and is not comparable to insurance premiums.[25][22] - **Distinguished** — Specialty property and casualty managing general agent and program administrator, placing insurance for carrier partners without retaining insurance risk. Specialty-insurance-distribution revenue of $57.7 million was about 1.5% of consolidated revenue; White Mountains acquired a controlling interest in September 2025.[26][22] - **Bamboo** — Technology- and data-enabled homeowners’ insurance distribution and MGA platform, principally in California and also Texas from the third quarter of 2025. Its commission and fee revenue for January 1 through December 5 was $211 million, about 5.7% of consolidated revenue; this is a partial-year figure.[27]+2 - **Other Operations** — A broad grouping that includes investment assets and various businesses and interests, rather than a single comparable operating line. Its $1,230.2 million of revenue was about 32.9% of consolidated revenue, but included a large sale-related gain, so that share is not a clean measure of ongoing operating contribution.[1]+2 [1]+11

Company history

White Mountains traces its corporate heritage to Jack Byrne’s turnaround of Fireman’s Fund: Byrne took the insurer public in 1985, and it was sold to Allianz in 1991. White Mountains itself expanded into reinsurance with its initial 50% investment in Folksamerica Re in 1994, acquiring the remaining stake in 1998. In 1999, its predecessor was reorganized under the White Mountains Insurance Group name.[30][31]

A major expansion followed in 2001, when White Mountains acquired CGU’s U.S. property-and-casualty operations and adopted OneBeacon as the business name. The company sold Esurance and Answer Financial to Allstate in 2011, then exited other major businesses: it sold Sirius Group in 2016 and OneBeacon to Intact in 2017.[32]+3

White Mountains broadened its portfolio with a controlling investment in NSM in 2018 and a stake in asset manager Kudu, then acquired a controlling interest in Ark in 2021. It sold NSM in 2022. In 2024, it acquired control of Bamboo; in December 2025, it sold most of its interest to CVC-affiliated buyers while retaining a smaller indirect stake.[36]+5 [34]+11

Sources

• WHITE MOUNTAINS INSURANCE GROUP LTD — Full financial report (2025 Y)
  25 citations

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• www.whitemountains.com[4]
• www.whitemountains.com[5]
• www.whitemountains.com[6]
• WHITE MOUNTAINS INSURANCE GROUP LTD — Full financial report (2026 Q 2)[9]
• arkunderwriting.com[11]
• www.whitemountains.com[30]
• www.sec.gov[31]
• www.whitemountains.com[32]
• www.whitemountains.com[33]
• WHITE MOUNTAINS INSURANCE GROUP LTD — Full financial report (2017 Y)[34]
• WHITE MOUNTAINS INSURANCE GROUP LTD — Full financial report (2017 Q 3)[35]
• www.whitemountains.com[36]
• www.whitemountains.com[37]
• WHITE MOUNTAINS INSURANCE GROUP LTD — Full financial report (2021 Y)[38]
• www.sec.gov[39]
• www.whitemountains.com[40]