Significant Transactions	12 Months Ended
Dec. 31, 2019
Significant Transactions [Abstract]
Significant Transactions	Significant Transactions Acquisitions Elementum On May 31, 2019, White Mountains acquired a 30.0% limited partnership interest in Elementum, a third-party registered investment adviser specializing in natural catastrophe ILS, for $55.1 million (the “Elementum Transaction”). Elementum manages separate accounts and pooled investment vehicles across various ILS sectors, including catastrophe bonds, collateralized reinsurance investments and industry loss warranties, on behalf of third-party clients. As part of the Elementum Transaction, White Mountains also committed to invest $50.0 million in ILS funds managed by Elementum. As of December 31, 2019, White Mountains has invested $40.0 million into three Elementum-managed funds. White Mountains has elected to use the fair value option for its investment in Elementum. Both the investment in Elementum and the investments in ILS funds managed by Elementum are included within other long-term investments. NSM On May 11, 2018, White Mountains acquired a 95.0% basic unit ownership interest in NSM (83.6% on a fully diluted, fully converted basis). White Mountains funded the acquisition through a combination of cash on hand and new borrowings by NSM. White Mountains paid $274.2 million of cash consideration for its equity interest in NSM, and NSM borrowed $100.0 million in new debt as part of the transaction. During the third quarter of 2018, White Mountains recorded a purchase price adjustment of an additional $2.1 million of consideration, which was paid in the fourth quarter of 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 in the Other Operations segment, which were expensed in 2018. On May 18, 2018, NSM acquired 100% of Fresh Insurance Services Group Limited (“Fresh Insurance”). Fresh Insurance is an insurance broker that offers non-standard personal lines products in the United Kingdom. NSM paid $49.6 million of upfront cash consideration for Fresh Insurance. NSM borrowed $51.0 million to fund the transaction. During the three months ended March 31, 2019, NSM paid a purchase price adjustment of an additional $0.7 million of consideration. 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. On December 3, 2018, NSM acquired all the net assets of KBK Insurance Group, Inc. (“KBK”), a specialized MGU focused on the towing and transportation space. NSM paid $60.0 million of upfront cash consideration for KBK. White Mountains contributed $29.0 million and NSM borrowed $30.1 million to fund the transaction. White Mountains recognized $59.4 million of goodwill and other intangible assets, reflecting acquisition date fair values, for which the relative fair values of goodwill and other intangible assets had not yet been finalized as of December 31, 2018. During 2019, NSM determined that the relative values of goodwill and other intangible assets recorded in connection with the KBK transaction were $32.6 million and $32.7 million, reflecting acquisition date fair values. The purchase price is subject to additional adjustments based upon growth in EBITDA during three earn out periods ending in December 2019, December 2020 and December 2021. During the three months ended March 31, 2019, NSM recorded a purchase price adjustment of $5.9 million relating to the fair value of the contingent consideration earnout liability in connection with the acquisition. On April 1, 2019, NSM acquired 100% of Embrace Pet Insurance (“Embrace”), a nationwide provider of pet health insurance for dogs and cats. NSM paid $71.5 million of cash consideration, net of cash acquired, for Embrace. White Mountains contributed $58.2 million to NSM and NSM borrowed $20.4 million to fund the transaction. White Mountains recognized $52.2 million of goodwill and $15.4 million of other intangible assets, reflecting acquisition date fair values. On June 28, 2019, NSM acquired the renewal rights on its U.S. collector car business (the “Renewal Rights”) from American International Group, Inc. (“AIG”) for $82.5 million. The acquisition satisfied NSM’s obligation to acquire the Renewal Rights from AIG. See Note 19 — “Commitments and Contingencies”. White Mountains contributed $59.1 million to NSM and NSM borrowed $22.5 million to fund the transaction. White Mountains recognized $82.5 million of other intangible assets, reflecting the acquisition date fair value. See Note 4 — “Goodwill and Other Intangible Assets”. The contingent consideration earnout liabilities related to the NSM, Fresh Insurance and KBK acquisitions are subject to adjustments based upon EBITDA, EBITDA projections, and present value factors for acquired entities. For the year ended December 31, 2019, NSM recognized pre-tax expense of $2.1 million for the change in the fair value of its contingent consideration earnout liabilities. For the period from May 11, 2018 through December 31, 2018, NSM recognized pre-tax expense of $2.7 million for the change in the fair value of its contingent consideration earnout liabilities. Any future adjustments to contingent consideration earnout liabilities under the agreements will be recognized through pre-tax income. As of December 31, 2019 and 2018, NSM recognized total contingent consideration earnout liabilities of $20.6 million and $20.2 million. DavidShield On January 24, 2018, White Mountains acquired a 50% ownership interest in DavidShield, its joint venture partner in 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 reorganized 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% interest in 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 $125.0 million in Kudu in exchange for a 49.5% basic unit ownership interest in Kudu (42.7% on a fully diluted, fully converted basis). On April 4, 2019, White Mountains acquired the ownership interests in Kudu held by certain funds managed by Oaktree for cash consideration of $81.4 million. In addition, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total capital commitment to $250.0 million. White Mountains recognized total assets acquired of $155.5 million, including $7.6 million of goodwill and $2.2 million of other intangible assets, total liabilities assumed of $0.8 million and non-controlling interest of $1.5 million. As a result of the Kudu Transaction, White Mountains’s basic unit ownership of Kudu increased from 49.5% to 99.1% (42.7% to 85.4% on a fully diluted, fully converted basis), and White Mountains began consolidating Kudu as a reportable segment in its financial statements during the second quarter of 2019. White Mountains’s consolidated financial statements and its segment disclosures include Kudu’s results for the period from April 4, 2019 to December 31, 2019. For periods prior to the Kudu Transaction, White Mountains determined that Kudu was a VIE, but White Mountains was not the primary beneficiary. In those periods, White Mountains elected to use the fair value option. During the fourth quarter of 2019, White Mountains increased its total capital commitment to Kudu by $100.0 million to $350.0 million of which $129.0 million is undrawn as of December 31, 2019. Also during the fourth quarter of 2019, Kudu obtained a committed $125.0 million credit facility, of which $68.0 million was undrawn as of December 31, 2019. See Note 5 — “Debt”. Dispositions MediaAlpha On February 26, 2019, MediaAlpha completed the MediaAlpha Transaction. White Mountains received net cash proceeds of $89.3 million from the MediaAlpha Transaction. White Mountains recognized a realized gain of $67.5 million and reduced its ownership interest to 48.3% of the basic units outstanding of MediaAlpha (42.0% on a fully diluted, fully converted basis) as a result of the MediaAlpha Transaction. White Mountains’s remaining ownership interest in MediaAlpha no longer meets the criteria for a controlling ownership interest and, accordingly, White Mountains deconsolidated MediaAlpha on February 26, 2019. White Mountains’s consolidated statement of operations and comprehensive income and its segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. Upon deconsolidation, White Mountains’s investment in MediaAlpha met the criteria to be accounted for under the equity method or under the fair value option. See Note 15 — “Equity-Method Eligible Investments”. White Mountains elected the fair value option and the investment in MediaAlpha was initially measured at its estimated fair value of $114.7 million as of the transaction date, with the change in fair value of $114.7 million recognized as an unrealized investment gain. White Mountains recognized a total of $182.2 million of realized gain and unrealized investment gain on the MediaAlpha Transaction. OneBeacon On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon Insurance Group, Ltd. in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a net gain of $554.6 million, net of transaction costs, and other comprehensive income of $2.9 million. 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 20 — “Held for Sale and Discontinued Operations”. Tranzact On July 21, 2016, White Mountains completed the disposition of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million. During 2017, White Mountains recorded a $3.2 million increase to the gain from sale of Tranzact in discontinued operations as a result of a change in state tax expense. Sirius Group On April 18, 2016, White Mountains completed the disposition of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively “CMI”) for approximately $2.6 billion. During 2017, White Mountains recorded a $0.7 million reduction to the gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees. During 2019 and 2018, White Mountains recorded a gain (loss) of $0.8 million and $(17.3) million within net gain (loss) on sale of discontinued operations for a contingency related to the sale of Sirius Group. See Note 19 — “Commitments and Contingencies”.

On May 31, 2019, White Mountains acquired a 30.0% limited partnership interest in Elementum, a third-party registered investment adviser specializing in natural catastrophe ILS, for $55.1 million (the “Elementum Transaction”). Elementum manages separate accounts and pooled investment vehicles across various ILS sectors, including catastrophe bonds, collateralized reinsurance investments and industry loss warranties, on behalf of third-party clients. As part of the Elementum Transaction, White Mountains also committed to invest $50.0 million in ILS funds managed by Elementum. As of December 31, 2019, White Mountains has invested $40.0 million into three Elementum-managed funds.

On May 11, 2018, White Mountains acquired a 95.0% basic unit ownership interest in NSM (83.6% on a fully diluted, fully converted basis). White Mountains funded the acquisition through a combination of cash on hand and new borrowings by NSM. White Mountains paid $274.2 million of cash consideration for its equity interest in NSM, and NSM borrowed $100.0 million in new debt as part of the transaction. During the third quarter of 2018, White Mountains recorded a purchase price adjustment of an additional $2.1 million of consideration, which was paid in the fourth quarter of 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 in the Other Operations segment, which were expensed in 2018.

On May 18, 2018, NSM acquired 100% of Fresh Insurance Services Group Limited (“Fresh Insurance”). Fresh Insurance is an insurance broker that offers non-standard personal lines products in the United Kingdom. NSM paid $49.6 million of upfront cash consideration for Fresh Insurance. NSM borrowed $51.0 million to fund the transaction. During the three months ended March 31, 2019, NSM paid a purchase price adjustment of an additional $0.7 million of consideration. 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.

On December 3, 2018, NSM acquired all the net assets of KBK Insurance Group, Inc. (“KBK”), a specialized MGU focused on the towing and transportation space. NSM paid $60.0 million of upfront cash consideration for KBK. White Mountains contributed $29.0 million and NSM borrowed $30.1 million to fund the transaction. White Mountains recognized $59.4 million of goodwill and other intangible assets, reflecting acquisition date fair values, for which the relative fair values of goodwill and other intangible assets had not yet been finalized as of December 31, 2018. During 2019, NSM determined that the relative values of goodwill and other intangible assets recorded in connection with the KBK transaction were $32.6 million and $32.7 million, reflecting acquisition date fair values. The purchase price is subject to additional adjustments based upon growth in EBITDA during three earn out periods ending in December 2019, December 2020 and December 2021. During the three months ended March 31, 2019, NSM recorded a purchase price adjustment of $5.9 million relating to the fair value of the contingent consideration earnout liability in connection with the acquisition.

On April 1, 2019, NSM acquired 100% of Embrace Pet Insurance (“Embrace”), a nationwide provider of pet health insurance for dogs and cats. NSM paid $71.5 million of cash consideration, net of cash acquired, for Embrace. White Mountains contributed $58.2 million to NSM and NSM borrowed $20.4 million to fund the transaction. White Mountains recognized $52.2 million of goodwill and $15.4 million of other intangible assets, reflecting acquisition date fair values.

On June 28, 2019, NSM acquired the renewal rights on its U.S. collector car business (the “Renewal Rights”) from American International Group, Inc. (“AIG”) for $82.5 million. The acquisition satisfied NSM’s obligation to acquire the Renewal Rights from AIG. See Note 19 — “Commitments and Contingencies”. White Mountains contributed $59.1 million to NSM and NSM borrowed $22.5 million to fund the transaction. White Mountains recognized $82.5 million of other intangible assets, reflecting the acquisition date fair value. See Note 4 — “Goodwill and Other Intangible Assets”.

The contingent consideration earnout liabilities related to the NSM, Fresh Insurance and KBK acquisitions are subject to adjustments based upon EBITDA, EBITDA projections, and present value factors for acquired entities. For the year ended December 31, 2019, NSM recognized pre-tax expense of $2.1 million for the change in the fair value of its contingent consideration earnout liabilities. For the period from May 11, 2018 through December 31, 2018, NSM recognized pre-tax expense of $2.7 million for the change in the fair value of its contingent consideration earnout liabilities. Any future adjustments to contingent consideration earnout liabilities under the agreements will be recognized through pre-tax income. As of December 31, 2019 and 2018, NSM recognized total contingent consideration earnout liabilities of $20.6 million and $20.2 million.

On January 24, 2018, White Mountains acquired a 50% ownership interest in DavidShield, its joint venture partner in 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 reorganized 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% interest in DavidShield was $41.8 million, or $28.3 million net of the financing provided for the restructuring.

On February 5, 2018, White Mountains entered into an agreement to fund up to $125.0 million in Kudu in exchange for a 49.5% basic unit ownership interest in Kudu (42.7% on a fully diluted, fully converted basis). On April 4, 2019, White Mountains acquired the ownership interests in Kudu held by certain funds managed by Oaktree for cash consideration of $81.4 million. In addition, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total capital commitment to $250.0 million. White Mountains recognized total assets acquired of $155.5 million, including $7.6 million of goodwill and $2.2 million of other intangible assets, total liabilities assumed of $0.8 million and non-controlling interest of $1.5 million.

As a result of the Kudu Transaction, White Mountains’s basic unit ownership of Kudu increased from 49.5% to 99.1% (42.7% to 85.4% on a fully diluted, fully converted basis), and White Mountains began consolidating Kudu as a reportable segment in its financial statements during the second quarter of 2019. White Mountains’s consolidated financial statements and its segment disclosures include Kudu’s results for the period from April 4, 2019 to December 31, 2019. For periods prior to the Kudu Transaction, White Mountains determined that Kudu was a VIE, but White Mountains was not the primary beneficiary. In those periods, White Mountains elected to use the fair value option.

During the fourth quarter of 2019, White Mountains increased its total capital commitment to Kudu by $100.0 million to $350.0 million of which $129.0 million is undrawn as of December 31, 2019. Also during the fourth quarter of 2019, Kudu obtained a committed $125.0 million credit facility, of which $68.0 million was undrawn as of December 31, 2019. See Note 5 — “Debt”.

White Mountains recognized a realized gain of $67.5 million and reduced its ownership interest to 48.3% of the basic units outstanding of MediaAlpha (42.0% on a fully diluted, fully converted basis) as a result of the MediaAlpha Transaction. White Mountains’s remaining ownership interest in MediaAlpha no longer meets the criteria for a controlling ownership interest and, accordingly, White Mountains deconsolidated MediaAlpha on February 26, 2019. White Mountains’s consolidated statement of operations and comprehensive income and its segment disclosures include MediaAlpha’s results of operations for the period from January 1, 2019 through February 26, 2019. Upon deconsolidation, White Mountains’s investment in MediaAlpha met the criteria to be accounted for under the equity method or under the fair value option. See Note 15 — “Equity-Method Eligible Investments”. White Mountains elected the fair value option and the investment in MediaAlpha was initially measured at its estimated fair value of $114.7 million as of the transaction date, with the change in fair value of $114.7 million recognized as an unrealized investment gain. White Mountains recognized a total of $182.2 million of realized gain and unrealized investment gain on the MediaAlpha Transaction.

On September 28, 2017, Intact Financial Corporation completed its acquisition of OneBeacon Insurance Group, Ltd. in an all-cash transaction for $18.10 per share (the “OneBeacon Transaction”). White Mountains received $1.3 billion in cash proceeds from the OneBeacon Transaction and recorded a net gain of $554.6 million, net of transaction costs, and other comprehensive income of $2.9 million. 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 20 — “Held for Sale and Discontinued Operations”.

On July 21, 2016, White Mountains completed the disposition of Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC and received net proceeds of $221.3 million. During 2017, White Mountains recorded a $3.2 million increase to the gain from sale of Tranzact in discontinued operations as a result of a change in state tax expense.

On April 18, 2016, White Mountains completed the disposition of Sirius International Insurance Group, Ltd. (“Sirius Group”) to CM International Pte. Ltd. and CM Bermuda Limited (collectively “CMI”) for approximately $2.6 billion. During 2017, White Mountains recorded a $0.7 million reduction to the gain from sale of Sirius Group as a result of a change to the valuation of the accrued incentive compensation payable to Sirius Group employees. During 2019 and 2018, White Mountains recorded a gain (loss) of $0.8 million and $(17.3) million within net gain (loss) on sale of discontinued operations for a contingency related to the sale of Sirius Group. See Note 19 — “Commitments and Contingencies”.