Commitments and Contingencies	12 Months Ended
Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies White Mountains leases certain office space under non-cancellable operating leases that expired on various dates through 2021. Rental expense for all of White Mountains’ locations was $18.8 million, $22.8 million and $28.1 million for the years ended December 31, 2012, 2011 and 2010. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’ future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $14.8 million, $13.5 million, $11.8 million, and $33.3 million for 2013, 2014, 2015 and 2016 and thereafter, respectively. White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $124.7 million, do not have fixed funding dates. Capital Lease OBIC sold the majority of its fixed assets and capitalized software to OneBeacon Services LLC (“OB Services”) at a cost equal to book value with no gain or loss recorded on the sale. Subsequent to purchasing the fixed assets and capitalized software from OBIC, OB Services entered into lease financing arrangements with US Bancorp and Fifth Third whereby OB Services sold its furniture and equipment and its capitalized software, respectively, to US Bancorp and Fifth Third. The assets were sold at a cost equal to net book value. OB Services 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. OB Services received cash proceeds of $23.1 million as a result of entering into the sale-leaseback transactions. At the end of the lease terms, OB Services will be obligated to purchase the leased assets for a nominal fee, after which all rights, title and interest would transfer to OB Services. 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, 2012 and 2011, OB Services had a capital lease obligation of $18.2 million and $23.1 million, respectively, included within other liabilities and a capital lease asset of $16.1 million and $22.9 million included within other assets. The underlying assets will continue to be depreciated over their respective useful lives. OB Services’ future annual minimum rental payments are $5.3 million for each of the years ended December 31, 2013, 2014 and 2015 and $1.9 million for the year ended December 31, 2016. Assigned Risks As a condition of its license to do business in certain states, White Mountains’ 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’ 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. At December 31, 2012, the reserve for such assessments totaled $12.9 million. Esurance On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to Allstate pursuant 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 contingent liabilities under the Agreement as follows: (i) the final purchase price payable by Allstate under the Agreement, which is based upon the book value of the Transferred Subsidiaries at the closing date, is subject to a true-up process that has not yet been concluded, (ii) 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, (iii) the Company indemnifies Allstate for breaches of certain covenants in the Agreement, including certain agreements by the Company not to engage in certain competing business activities for two years after the closing and not to solicit certain employees of the Transferred Subsidiaries for three years after the closing, and (iv) 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), to be computed annually on the first three anniversaries of the closing date. 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’ 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’ financial condition, results of operations or cash flows. The following summarizes significant ongoing non-claims related litigation or arbitration as of December 31, 2012: Esurance Sale In 2011, the Company sold its Esurance and Answer Financial businesses (the “Transferred Companies”) to The Allstate Corporation (”Allstate”) for a purchase price of approximately $1.01 billion. The purchase price consisted of $700.0 million plus the tangible book value of the Transferred Companies at the closing, which was estimated to be $308.0 million. Following closing, Allstate was required to prepare a final closing statement, including an audited balance sheet for the Transferred Companies as of the closing date. The Company is disputing Allstate’s calculation of tangible book value in the closing statement. The amount in dispute is approximately $20.0 million, after tax. The dispute principally relates to (i) the elimination of $24.7 million (pre-tax) of deferred acquisition costs ($16.0 million, after tax) and (ii) the inclusion of a liability equal to the costs associated with an Esurance extra-contractual (“ECO”) matter settled in April 2012 of $5.2 million ($3.4 million, after tax). Per the agreement governing the sale of the Transferred Companies (the “Sale Agreement”), disputes over the closing statement are to be arbitrated by an independent accountant. The Company believes this final closing statement was required to be prepared and audited no later than January 5, 2012. Allstate did not deliver the final closing statement to the Company until June 6, 2012, with an audit report dated June 1, 2012. As a result, in addition to the substantive disputes over the final closing statement, the Company also believes that Allstate's failure to have the final closing statement prepared and audited by the required date constituted a breach of Allstate's obligations under the Sale Agreement. The Company brought suit in the United States District Court for the Southern District of New York in connection with such breach. The court concluded that the Company’s breach claim should also be arbitrated by the independent accountant under the Sale Agreement. That process is proceeding. 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, OBIC-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") and emerged from bankruptcy at the end of 2012 in a Chapter 11 reorganization. During the bankruptcy proceedings, the Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions. 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 million for Tribune common stock tendered in connection with the LBO. In December 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate all of the Noteholder Actions for pretrial matters and transfer all such proceedings to the United States District Court for the Southern District of New York. In addition, OneBeacon, OBIC-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, 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”). The Committee Action has since been consolidated with the Noteholder Actions. In September 2012, a case management order was entered in the consolidated cases, setting forth, among other things, a briefing schedule for an omnibus motion to dismiss in the Noteholder Actions. The court is expected to hear oral argument on that motion in March 2013. Discovery and other motion practice (other than motions to amend the complaints) in the Committee Action and the Noteholder Actions is stayed until further order of the court. Ace American Insurance Company A subsidiary of OneBeacon, OBH, was sued in Federal Court in the Eastern District of Pennsylvania on August 17, 2012 by Ace American Insurance Company (“Ace”). The complaint alleges that OBH, through a professional recruiting firm, improperly hired a group of Ace employees from Ace's surety division. The complaint sought injunctive relief and unspecified damages. After court-ordered expedited discovery was completed, the claims for injunctive relief were resolved pursuant to a confidential agreement. The remaining claim against OBH is for damages only and is scheduled to be heard in April. After the claims against OBH for injunctive relief were resolved, Ace filed a Demand for Arbitration against five of the former Ace surety employees hired by OneBeacon, alleging breach of their duty of loyalty to Ace and misappropriation of Ace trade secrets. OneBeacon believes that Ace's damages claim against OBH and the claims against the individual employees are without merit and intends to vigorously defend both

Commitments and Contingencies

White Mountains leases certain office space under non-cancellable operating leases that expired on various dates through 2021. Rental expense for all of White Mountains’ locations was $18.8 million, $22.8 million and $28.1 million for the years ended December 31, 2012, 2011 and 2010. White Mountains also has various other lease obligations that are immaterial in the aggregate. White Mountains’ future annual minimum rental payments required under non-cancellable leases, which are primarily for office space, are $14.8 million, $13.5 million, $11.8 million, and $33.3 million for 2013, 2014, 2015 and 2016 and thereafter, respectively.

White Mountains also has future binding commitments to fund certain other-long term investments. These commitments, which total $124.7 million, do not have fixed funding dates.

Capital Lease

OBIC sold the majority of its fixed assets and capitalized software to OneBeacon Services LLC (“OB Services”) at a cost equal to book value with no gain or loss recorded on the sale. Subsequent to purchasing the fixed assets and capitalized software from OBIC, OB Services entered into lease financing arrangements with US Bancorp and Fifth Third whereby OB Services sold its furniture and equipment and its capitalized software, respectively, to US Bancorp and Fifth Third. The assets were sold at a cost equal to net book value. OB Services 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. OB Services received cash proceeds of $23.1 million as a result of entering into the sale-leaseback transactions. At the end of the lease terms, OB Services will be obligated to purchase the leased assets for a nominal fee, after which all rights, title and interest would transfer to OB Services. 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, 2012 and 2011, OB Services had a capital lease obligation of $18.2 million and $23.1 million, respectively, included within other liabilities and a capital lease asset of $16.1 million and $22.9 million included within other assets. The underlying assets will continue to be depreciated over their respective useful lives. OB Services’ future annual minimum rental payments are $5.3 million for each of the years ended December 31, 2013, 2014 and 2015 and $1.9 million for the year ended December 31, 2016.

Assigned Risks

As a condition of its license to do business in certain states, White Mountains’ 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’ 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. At December 31, 2012, the reserve for such assessments totaled $12.9 million.

Esurance

On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to Allstate pursuant 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 contingent liabilities under the Agreement as follows: (i) the final purchase price payable by Allstate under the Agreement, which is based upon the book value of the Transferred Subsidiaries at the closing date, is subject to a true-up process that has not yet been concluded, (ii) 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, (iii) the Company indemnifies Allstate for breaches of certain covenants in the Agreement, including certain agreements by the Company not to engage in certain competing business activities for two years after the closing and not to solicit certain employees of the Transferred Subsidiaries for three years after the closing, and (iv) 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), to be computed annually on the first three anniversaries of the closing date.

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’ 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’ financial condition, results of operations or cash flows.

The following summarizes significant ongoing non-claims related litigation or arbitration as of December 31, 2012:

Esurance Sale

In 2011, the Company sold its Esurance and Answer Financial businesses (the “Transferred Companies”) to The Allstate Corporation (”Allstate”) for a purchase price of approximately $1.01 billion. The purchase price consisted of $700.0 million plus the tangible book value of the Transferred Companies at the closing, which was estimated to be $308.0 million. Following closing, Allstate was required to prepare a final closing statement, including an audited balance sheet for the Transferred Companies as of the closing date. The Company is disputing Allstate’s calculation of tangible book value in the closing statement. The amount in dispute is approximately $20.0 million, after tax. The dispute principally relates to (i) the elimination of $24.7 million (pre-tax) of deferred acquisition costs ($16.0 million, after tax) and (ii) the inclusion of a liability equal to the costs associated with an Esurance extra-contractual (“ECO”) matter settled in April 2012 of $5.2 million ($3.4 million, after tax). Per the agreement governing the sale of the Transferred Companies (the “Sale Agreement”), disputes over the closing statement are to be arbitrated by an independent accountant.

The Company believes this final closing statement was required to be prepared and audited no later than January 5, 2012. Allstate did not deliver the final closing statement to the Company until June 6, 2012, with an audit report dated June 1, 2012. As a result, in addition to the substantive disputes over the final closing statement, the Company also believes that Allstate's failure to have the final closing statement prepared and audited by the required date constituted a breach of Allstate's obligations under the Sale Agreement. The Company brought suit in the United States District Court for the Southern District of New York in connection with such breach. The court concluded that the Company’s breach claim should also be arbitrated by the independent accountant under the Sale Agreement. That process is proceeding.

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, OBIC-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") and emerged from bankruptcy at the end of 2012 in a Chapter 11 reorganization. During the bankruptcy proceedings, the Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions. 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 million for Tribune common stock tendered in connection with the LBO.

In December 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate all of the Noteholder Actions for pretrial matters and transfer all such proceedings to the United States District Court for the Southern District of New York.

In addition, OneBeacon, OBIC-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, 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”). The Committee Action has since been consolidated with the Noteholder Actions.

In September 2012, a case management order was entered in the consolidated cases, setting forth, among other things, a briefing schedule for an omnibus motion to dismiss in the Noteholder Actions. The court is expected to hear oral argument on that motion in March 2013. Discovery and other motion practice (other than motions to amend the complaints) in the Committee Action and the Noteholder Actions is stayed until further order of the court.

Ace American Insurance Company

A subsidiary of OneBeacon, OBH, was sued in Federal Court in the Eastern District of Pennsylvania on August 17, 2012 by Ace American Insurance Company (“Ace”). The complaint alleges that OBH, through a professional recruiting firm, improperly hired a group of Ace employees from Ace's surety division. The complaint sought injunctive relief and unspecified damages. After court-ordered expedited discovery was completed, the claims for injunctive relief were resolved pursuant to a confidential agreement. The remaining claim against OBH is for damages only and is scheduled to be heard in April. After the claims against OBH for injunctive relief were resolved, Ace filed a Demand for Arbitration against five of the former Ace surety employees hired by OneBeacon, alleging breach of their duty of loyalty to Ace and misappropriation of Ace trade secrets. OneBeacon believes that Ace's damages claim against OBH and the claims against the individual employees are without merit and intends to vigorously defend both