Commitments and Contingencies	12 Months Ended
Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies White Mountains leases certain office space under non-cancellable operating leases that expire on various dates through 2021. Rental expense for all of White Mountains’s locations was $15.5 million, $11.8 million and $12.1 million for the years ended December 31, 2015, 2014 and 2013. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’s future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $14.3 million, $13.4 million, $11.4 million, and $50.6 million for 2016, 2017, 2018 and 2019 and thereafter, respectively. White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $82.2 million, do not have fixed funding dates. Capital Lease In December 2011, OneBeacon sold the majority of its fixed assets and capitalized software. OneBeacon entered into lease financing arrangements with US Bancorp and Fifth Third whereby OneBeacon sold furniture and equipment and capitalized software, respectively, at a cost equal to net book value. OneBeacon then leased the fixed assets back from US Bancorp for a lease term of five years and leased the capitalized software back from Fifth Third for a lease term of four years. OneBeacon received cash proceeds of $23.1 million as a result of entering into the sale-leaseback transactions. In December 2015, the lease agreement with Fifth Third expired and OneBeacon purchased the leased assets under the agreement with a remaining book value of $1.3 million for a nominal fee. At the end of the lease term with US Bancorp on December 31, 2016, OneBeacon will purchase the leased furniture and equipment assets for a nominal fee, and all rights, title and interest will transfer back to it. As of December 31, 2015 and 2014, OneBeacon had a capital lease obligation of $1.7 million and $7.1 million, respectively, included within other liabilities and a capital lease asset of $2.4 million and $7.0 million included within other assets. The underlying assets will continue to be depreciated over their respective useful lives. OneBeacon’s future annual minimum rental payments are $1.7 million for the year ended December 31, 2016. Assigned Risks As a condition of its license to do business in certain states, White Mountains’s insurance operations are required to participate in mandatory shared market mechanisms. Each state dictates the types of insurance and the level of coverage that must be provided. The total amount of business an insurer is required to accept is based on its market share of voluntary business in the state. In certain cases, White Mountains is obligated to write business from mandatory shared market mechanisms at some time in the future based on the market share of voluntary policies it is currently writing. Underwriting results related to assigned risk plans are typically adverse and are not subject to the predictability associated with White Mountains’s voluntarily written business. Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains accrues any significant insolvencies when the loss is probable and the assessment amount can be reasonably estimated. The actual amount of such assessments will depend upon the final outcome of rehabilitation proceedings and will be paid over several years. As of December 31, 2015, the reserve for such assessments totaled $8.5 million. Legal Contingencies White Mountains, and the insurance and reinsurance industry in general, are routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3 - “Reserves for Unpaid Losses and Loss Adjustment Expenses”. White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or it there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of December 31, 2015: Esurance On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011 (the “Agreement”). Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) specified litigation matters, (b) losses of the Transferred Subsidiaries arising from extra-contractual claims and claims in excess of policy limits (“ECO/EPL losses”), (c) certain corporate reorganizations effected to remove entities from the Transferred Subsidiaries that were not being sold in the transaction, and (d) certain tax matters, including certain net operating losses being less than stated levels. In addition, the Company retained 90% of positive or negative development in the loss reserves of the Transferred Subsidiaries as of the closing date (net of ECO/EPL losses) through December 31, 2014 (the “Reserve Settlement”). Runoff Transaction Subsequent to the closing of the Runoff Transaction, on January 22, 2015, three holders of insurance policies issued by the companies OneBeacon sold to Armour in the Runoff Transaction filed a Petition for Review with the Commonwealth Court of Pennsylvania (Commonwealth Court) requesting that the Commonwealth Court vacate the Pennsylvania Insurance Department’s (Department) orders approving the Runoff Transaction and denying their right to intervene in the Department’s regulatory review of the Runoff Transaction. On January 14, 2016, the Commonwealth Court upheld the Department’s December 23, 2014 order approving the Runoff Transaction. The objectors did not appeal the Commonwealth Court's decision and the time for appeal has expired. Tribune Company In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon received approximately $32.0 million and Sirius Group, which is now included in discontinued operations, received $6.1 million for Tribune common stock tendered in connection with the LBO. The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs’ appeal is pending. In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the United States District Court for the Southern District of New York and was stayed pending the motion to dismiss in the Noteholder Action. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014. No amount has been accrued in connection with this matter as of December 31, 2015, as the amount of loss, if any, cannot be reasonably estimated. OneTitle White Mountains holds a 20% ownership interest in OneTitle Holdings LLC (“OTH”) and has provided a $10.0 million surplus note facility under which OTH’s wholly-owned insurance subsidiary, OneTitle National Guaranty Company, Inc. may draw funds. At December 31, 2015, no funds had been drawn on the surplus note facility.

Commitments and Contingencies

White Mountains leases certain office space under non-cancellable operating leases that expire on various dates through 2021. Rental expense for all of White Mountains’s locations was $15.5 million, $11.8 million and $12.1 million for the years ended December 31, 2015, 2014 and 2013. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’s future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $14.3 million, $13.4 million, $11.4 million, and $50.6 million for 2016, 2017, 2018 and 2019 and thereafter, respectively.

White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $82.2 million, do not have fixed funding dates.

Capital Lease

In December 2011, OneBeacon sold the majority of its fixed assets and capitalized software. OneBeacon entered into lease financing arrangements with US Bancorp and Fifth Third whereby OneBeacon sold furniture and equipment and capitalized software, respectively, at a cost equal to net book value. OneBeacon then leased the fixed assets back from US Bancorp for a lease term of five years and leased the capitalized software back from Fifth Third for a lease term of four years. OneBeacon received cash proceeds of $23.1 million as a result of entering into the sale-leaseback transactions.

In December 2015, the lease agreement with Fifth Third expired and OneBeacon purchased the leased assets under the agreement with a remaining book value of $1.3 million for a nominal fee. At the end of the lease term with US Bancorp on December 31, 2016, OneBeacon will purchase the leased furniture and equipment assets for a nominal fee, and all rights, title and interest will transfer back to it.

As of December 31, 2015 and 2014, OneBeacon had a capital lease obligation of $1.7 million and $7.1 million, respectively, included within other liabilities and a capital lease asset of $2.4 million and $7.0 million included within other assets. The underlying assets will continue to be depreciated over their respective useful lives. OneBeacon’s future annual minimum rental payments are $1.7 million for the year ended December 31, 2016.

Assigned Risks

As a condition of its license to do business in certain states, White Mountains’s insurance operations are required to participate in mandatory shared market mechanisms. Each state dictates the types of insurance and the level of coverage that must be provided. The total amount of business an insurer is required to accept is based on its market share of voluntary business in the state. In certain cases, White Mountains is obligated to write business from mandatory shared market mechanisms at some time in the future based on the market share of voluntary policies it is currently writing. Underwriting results related to assigned risk plans are typically adverse and are not subject to the predictability associated with White Mountains’s voluntarily written business.

Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. White Mountains accrues any significant insolvencies when the loss is probable and the assessment amount can be reasonably estimated. The actual amount of such assessments will depend upon the final outcome of rehabilitation proceedings and will be paid over several years. As of December 31, 2015, the reserve for such assessments totaled $8.5 million.

Legal Contingencies

White Mountains, and the insurance and reinsurance industry in general, are routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3 - “Reserves for Unpaid Losses and Loss Adjustment Expenses”.

White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or it there is a reasonable possibility that a loss may have been incurred.

Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows.

The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of December 31, 2015:

Esurance

On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011 (the “Agreement”). Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) specified litigation matters, (b) losses of the Transferred Subsidiaries arising from extra-contractual claims and claims in excess of policy limits (“ECO/EPL losses”), (c) certain corporate reorganizations effected to remove entities from the Transferred Subsidiaries that were not being sold in the transaction, and (d) certain tax matters, including certain net operating losses being less than stated levels. In addition, the Company retained 90% of positive or negative development in the loss reserves of the Transferred Subsidiaries as of the closing date (net of ECO/EPL losses) through December 31, 2014 (the “Reserve Settlement”).

Runoff Transaction

Subsequent to the closing of the Runoff Transaction, on January 22, 2015, three holders of insurance policies issued by the companies OneBeacon sold to Armour in the Runoff Transaction filed a Petition for Review with the Commonwealth Court of Pennsylvania (Commonwealth Court) requesting that the Commonwealth Court vacate the Pennsylvania Insurance Department’s (Department) orders approving the Runoff Transaction and denying their right to intervene in the Department’s regulatory review of the Runoff Transaction. On January 14, 2016, the Commonwealth Court upheld the Department’s December 23, 2014 order approving the Runoff Transaction. The objectors did not appeal the Commonwealth Court's decision and the time for appeal has expired.

Tribune Company

In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon received approximately $32.0 million and Sirius Group, which is now included in discontinued operations, received $6.1 million for Tribune common stock tendered in connection with the LBO.

The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs’ appeal is pending.

In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the United States District Court for the Southern District of New York and was stayed pending the motion to dismiss in the Noteholder Action. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014. No amount has been accrued in connection with this matter as of December 31, 2015, as the amount of loss, if any, cannot be reasonably estimated.

OneTitle

White Mountains holds a 20% ownership interest in OneTitle Holdings LLC (“OTH”) and has provided a $10.0 million surplus note facility under which OTH’s wholly-owned insurance subsidiary, OneTitle National Guaranty Company, Inc. may draw funds. At December 31, 2015, no funds had been drawn on the surplus note facility.