Commitments and Contingencies	12 Months Ended
Dec. 31, 2014
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 $19.4 million, $20.2 million and $18.8 million for the years ended December 31, 2014, 2013 and 2012. 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 $16.5 million, $15.2 million, $13.6 million, and $54.7 million for 2015, 2016, 2017 and 2018 and thereafter, respectively. White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $81.7 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. At the end of the lease terms, OneBeacon will have the obligation to purchase the leased assets for a nominal fee, after which all rights, title and interest would transfer back. In accordance with ASC 840, OBIC recorded the sale of the assets with no gain or loss recognized while OB Services has recorded a capital lease obligation and a capital lease asset. As of December 31, 2014 and 2013, OneBeacon had a capital lease obligation of $7.1 million and $12.5 million, respectively, included within other liabilities and a capital lease asset of $7.0 million and $10.9 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 $5.3 million for the year ended December 31, 2015 and $1.8 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, 2014, the reserve for such assessments totaled $11.9 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. 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, 2014: Esurance On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to Allstate pursuant to a Stock Purchase Agreement dated as of May 17, 2011 (filed as an exhibit to the Company’s current report on Form 8-K on May 18, 2011, the “Agreement”). The Company has certain contingencies under the Agreement as follows: (i) subject to specified thresholds and limits, the Company generally indemnifies Allstate for breaches of its representations and warranties in the Agreement for a period of eighteen months (although longer for specified representations and warranties) from the closing, (ii) the Company indemnifies Allstate for breaches of certain covenants in the Agreement, including certain agreements by the Company not to solicit certain employees of the Transferred Subsidiaries for three years after the closing, and (iii) subject to specified thresholds and limits, the Company indemnifies 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 retains 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. 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 filed a Petition for Review with the Commonwealth Court of Pennsylvania (“Commonwealth Court”) requesting that the Commonwealth Court vacate the Pennsylvania Insurance Department’s (“PID”) orders approving the Runoff Transaction and denying their right to intervene in the PID’s regulatory review of the Runoff Transaction. White Mountains believes the claims made by the petitioners are without merit and intends to intervene in the proceedings before the Commonwealth Court to vigorously defend the propriety of the PID’s orders in their entirety. White Mountains believes that the possibility is remote that these proceedings could result in an adverse outcome or have a material financial impact on the Company’s results of operations or financial position in the future. 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 has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. Certain subsidiaries of White Mountains received approximately $39.0 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, 2014, as the amount of loss, if any, cannot be reasonably estimated.

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 $19.4 million, $20.2 million and $18.8 million for the years ended December 31, 2014, 2013 and 2012. 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 $16.5 million, $15.2 million, $13.6 million, and $54.7 million for 2015, 2016, 2017 and 2018 and thereafter, respectively.

White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $81.7 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. At the end of the lease terms, OneBeacon will have the obligation to purchase the leased assets for a nominal fee, after which all rights, title and interest would transfer back. In accordance with ASC 840, OBIC recorded the sale of the assets with no gain or loss recognized while OB Services has recorded a capital lease obligation and a capital lease asset. As of December 31, 2014 and 2013, OneBeacon had a capital lease obligation of $7.1 million and $12.5 million, respectively, included within other liabilities and a capital lease asset of $7.0 million and $10.9 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 $5.3 million for the year ended December 31, 2015 and $1.8 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, 2014, the reserve for such assessments totaled $11.9 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.

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, 2014:

Esurance

On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to Allstate pursuant to a Stock Purchase Agreement dated as of May 17, 2011 (filed as an exhibit to the Company’s current report on Form 8-K on May 18, 2011, the “Agreement”). The Company has certain contingencies under the Agreement as follows: (i) subject to specified thresholds and limits, the Company generally indemnifies Allstate for breaches of its representations and warranties in the Agreement for a period of eighteen months (although longer for specified representations and warranties) from the closing, (ii) the Company indemnifies Allstate for breaches of certain covenants in the Agreement, including certain agreements by the Company not to solicit certain employees of the Transferred Subsidiaries for three years after the closing, and (iii) subject to specified thresholds and limits, the Company indemnifies 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 retains 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.

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 filed a Petition for Review with the Commonwealth Court of Pennsylvania (“Commonwealth Court”) requesting that the Commonwealth Court vacate the Pennsylvania Insurance Department’s (“PID”) orders approving the Runoff Transaction and denying their right to intervene in the PID’s regulatory review of the Runoff Transaction. White Mountains believes the claims made by the petitioners are without merit and intends to intervene in the proceedings before the Commonwealth Court to vigorously defend the propriety of the PID’s orders in their entirety. White Mountains believes that the possibility is remote that these proceedings could result in an adverse outcome or have a material financial impact on the Company’s results of operations or financial position in the future.

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 has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. Certain subsidiaries of White Mountains received approximately $39.0 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, 2014, as the amount of loss, if any, cannot be reasonably estimated.