Significant Transactions	9 Months Ended
Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions Acquisitions NSM On May 11, 2018, White Mountains closed its acquisition of 95% of NSM for cash consideration of $274.2 million. White Mountains recognized a purchase price adjustment of $2.1 million during the three months ended September 30, 2018 that was paid in October 2018. White Mountains recognized total assets acquired related to NSM of $495.2 million, including $383.0 million of goodwill and other intangible assets, total liabilities assumed of $204.6 million, including contingent consideration earnout liabilities related to NSM’s previous acquisitions of its U.K.-based operations, of $10.2 million, and non-controlling interest of $14.4 million reflecting acquisition date fair values. In connection with the acquisition, White Mountains incurred transaction costs of $6.3 million, which were expensed in the second quarter of 2018. On May 18, 2018, NSM acquired 100% of Fresh Insurance Services Group Limited (“Fresh Insurance”), an insurance broker that specializes in non-standard personal lines products, motor trade, and van insurance in the United Kingdom, for cash consideration of $49.6 million. NSM recognized a purchase price adjustment of $0.7 million during the three months ended September 30, 2018. The purchase price is subject to additional adjustments based upon growth in EBITDA during two earnout periods, ending in February 2020 and February 2022. NSM recognized total assets acquired related to Fresh Insurance of $72.6 million, including $54.6 million of goodwill and other intangible assets, and total liabilities assumed of $22.3 million, reflecting acquisition date fair values. In connection with the acquisition, NSM recorded a contingent consideration earnout liability of $7.5 million. The contingent consideration earnout liabilities related to these acquisitions are subject to adjustment based upon EBITDA, EBITDA projections, and present value factors for acquired entities. For both the three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018, NSM recognized pre-tax expense of $2.6 million for the change in the fair value of its contingent consideration earnout liabilities for both Fresh Insurance and its other U.K.-based operations. Any future adjustments to contingent consideration earnout liabilities under the agreements will also be recognized through pre-tax income. As of September 30, 2018, NSM recorded contingent consideration earnout liabilities of $20.1 million. DavidShield On January 24, 2018, White Mountains acquired 50% of DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”), its joint venture partner in PassportCard Limited (“PassportCard”). DavidShield is a managing general agency that is the leading provider of expatriate medical insurance in Israel and uses the same card-based delivery system as PassportCard. As part of the transaction, White Mountains restructured its equity stake in PassportCard so that White Mountains and its partner in DavidShield would each own 50% of both businesses. To facilitate the transaction, White Mountains provided financing to its partner in the form of a non-interest bearing loan that is secured by the partner’s equity in PassportCard and DavidShield. The gross purchase price for the 50% of DavidShield was $41.8 million, or $28.3 million net of the financing provided for the restructuring. Kudu On February 5, 2018, White Mountains entered into an agreement to fund up to $127.5 million in Kudu Investment Management, LLC (“Kudu”), a capital provider to asset management and wealth management firms, including $125.0 million for new transactions. Kudu specializes in providing capital solutions to asset managers and registered investment advisers, including generational ownership transfers, management buyouts, acquisition and growth finance, as well as liquidity for legacy partners. As of September 30, 2018, White Mountains has funded $12.3 million in Kudu. Kudu closed their first transaction in the third quarter of 2018 and announced two more in October, deploying $63.0 million of capital in total, $31.5 million of which is from White Mountains. White Mountains has determined that Kudu is a VIE, however White Mountains is not the primary beneficiary. White Mountains has elected to take the fair value option for its investment in Kudu. MediaAlpha On October 5, 2017, MediaAlpha acquired certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com for an aggregate purchase price of $28.0 million. The majority of assets acquired, which are included in other intangible assets, consist of customer relationships, a non-compete agreement from the seller and domain names. See Note 4 — “Goodwill and Other Intangibles Assets”. On October 5, 2017, White Mountains acquired 131,579 newly-issued Class A common units of MediaAlpha for $12.5 million. As of September 30, 2018 and December 31, 2017 White Mountains’s ownership share in MediaAlpha was 62.2% and 64.4%. Dispositions OneBeacon On September 28, 2017, White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a gain of $554.6 million, net of transaction costs. As a result of the OneBeacon Transaction, OneBeacon’s results have been reported as discontinued operations within White Mountains’s GAAP financial statements. See Note 16 — “Held for Sale and Discontinued Operations”. Star & Shield On March 7, 2017, White Mountains completed the sale of Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”) and its investment in Star & Shield Insurance Exchange (“SSIE”) surplus notes to K2 Insurance Services, LLC. White Mountains did not recognize any gain or loss on the sale.

Significant Transactions

Acquisitions

NSM

On May 11, 2018, White Mountains closed its acquisition of 95% of NSM for cash consideration of $274.2 million. White Mountains recognized a purchase price adjustment of $2.1 million during the three months ended September 30, 2018 that was paid in October 2018. White Mountains recognized total assets acquired related to NSM of $495.2 million, including $383.0 million of goodwill and other intangible assets, total liabilities assumed of $204.6 million, including contingent consideration earnout liabilities related to NSM’s previous acquisitions of its U.K.-based operations, of $10.2 million, and non-controlling interest of $14.4 million reflecting acquisition date fair values. In connection with the acquisition, White Mountains incurred transaction costs of $6.3 million, which were expensed in the second quarter of 2018.

On May 18, 2018, NSM acquired 100% of Fresh Insurance Services Group Limited (“Fresh Insurance”), an insurance broker that specializes in non-standard personal lines products, motor trade, and van insurance in the United Kingdom, for cash consideration of $49.6 million. NSM recognized a purchase price adjustment of $0.7 million during the three months ended September 30, 2018. The purchase price is subject to additional adjustments based upon growth in EBITDA during two earnout periods, ending in February 2020 and February 2022. NSM recognized total assets acquired related to Fresh Insurance of $72.6 million, including $54.6 million of goodwill and other intangible assets, and total liabilities assumed of $22.3 million, reflecting acquisition date fair values. In connection with the acquisition, NSM recorded a contingent consideration earnout liability of $7.5 million.

The contingent consideration earnout liabilities related to these acquisitions are subject to adjustment based upon EBITDA, EBITDA projections, and present value factors for acquired entities. For both the three months ended September 30, 2018 and the period from May 11, 2018 through September 30, 2018, NSM recognized pre-tax expense of $2.6 million for the change in the fair value of its contingent consideration earnout liabilities for both Fresh Insurance and its other U.K.-based operations. Any future adjustments to contingent consideration earnout liabilities under the agreements will also be recognized through pre-tax income. As of September 30, 2018, NSM recorded contingent consideration earnout liabilities of $20.1 million.

DavidShield

On January 24, 2018, White Mountains acquired 50% of DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”), its joint venture partner in PassportCard Limited (“PassportCard”). DavidShield is a managing general agency that is the leading provider of expatriate medical insurance in Israel and uses the same card-based delivery system as PassportCard. As part of the transaction, White Mountains restructured its equity stake in PassportCard so that White Mountains and its partner in DavidShield would each own 50% of both businesses. To facilitate the transaction, White Mountains provided financing to its partner in the form of a non-interest bearing loan that is secured by the partner’s equity in PassportCard and DavidShield. The gross purchase price for the 50% of DavidShield was $41.8 million, or $28.3 million net of the financing provided for the restructuring.

Kudu

On February 5, 2018, White Mountains entered into an agreement to fund up to $127.5 million in Kudu Investment Management, LLC (“Kudu”), a capital provider to asset management and wealth management firms, including $125.0 million for new transactions. Kudu specializes in providing capital solutions to asset managers and registered investment advisers, including generational ownership transfers, management buyouts, acquisition and growth finance, as well as liquidity for legacy partners.

As of September 30, 2018, White Mountains has funded $12.3 million in Kudu. Kudu closed their first transaction in the third quarter of 2018 and announced two more in October, deploying $63.0 million of capital in total, $31.5 million of which is from White Mountains. White Mountains has determined that Kudu is a VIE, however White Mountains is not the primary beneficiary. White Mountains has elected to take the fair value option for its investment in Kudu.

MediaAlpha

On October 5, 2017, MediaAlpha acquired certain assets associated with the Health, Life and Medicare insurance business of Healthplans.com for an aggregate purchase price of $28.0 million. The majority of assets acquired, which are included in other intangible assets, consist of customer relationships, a non-compete agreement from the seller and domain names. See Note 4 — “Goodwill and Other Intangibles Assets”.

On October 5, 2017, White Mountains acquired 131,579 newly-issued Class A common units of MediaAlpha for $12.5 million. As of September 30, 2018 and December 31, 2017 White Mountains’s ownership share in MediaAlpha was 62.2% and 64.4%.

Dispositions

OneBeacon

On September 28, 2017, White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a gain of $554.6 million, net of transaction costs. As a result of the OneBeacon Transaction, OneBeacon’s results have been reported as discontinued operations within White Mountains’s GAAP financial statements. See Note 16 — “Held for Sale and Discontinued Operations”.

Star & Shield

On March 7, 2017, White Mountains completed the sale of Star & Shield Services LLC, Star & Shield Risk Management LLC, and Star & Shield Claims Services LLC (collectively “Star & Shield”) and its investment in Star & Shield Insurance Exchange (“SSIE”) surplus notes to K2 Insurance Services, LLC. White Mountains did not recognize any gain or loss on the sale.