Significant Transactions	3 Months Ended
Mar. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Transactions	Significant Transactions 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. Through December 31, 2016, Star & Shield’s assets and liabilities are reported as held for sale within White Mountains’s GAAP financial statements. See Note 16 — “Held for Sale and Discontinued Operations”. Acquisitions The following acquisitions are included in White Mountains’s consolidated financial statements from the date of acquisition. The assets acquired and liabilities assumed have been measured at their acquisition date fair values. 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 leading capital provider to asset management and wealth management firms. 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 March 31, 2018, White Mountains has funded $1.8 million in Kudu. 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 March 31, 2018 and December 31, 2017 White Mountains’s ownership share in MediaAlpha was 62.3% and 64.4%.

Significant Transactions

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. Through December 31, 2016, Star & Shield’s assets and liabilities are reported as held for sale within White Mountains’s GAAP financial statements. See Note 16 — “Held for Sale and Discontinued Operations”.

Acquisitions

The following acquisitions are included in White Mountains’s consolidated financial statements from the date of acquisition. The assets acquired and liabilities assumed have been measured at their acquisition date fair values.

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 leading capital provider to asset management and wealth management firms. 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 March 31, 2018, White Mountains has funded $1.8 million in Kudu. 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 March 31, 2018 and December 31, 2017 White Mountains’s ownership share in MediaAlpha was 62.3% and 64.4%.