Significant Transactions	12 Months Ended
Dec. 31, 2012
Business Combinations [Abstract]
Significant Transactions	Significant Transactions Formation of HG Global and BAM In 2012, White Mountains capitalized HG Global with $594.5 million to fund BAM, a newly formed mutual municipal bond insurer. As of December 31, 2012, White Mountains owned 97.3% of HG Global’s preferred equity and 88.7% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM Surplus Notes. Through HG Re, which had statutory capital of $412.0 million at December 31, 2012, HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding, on a per policy basis. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time. For the year ended December 31, 2012, HG Global had pre-tax income of $14.2 million, which included $18.4 million of interest income on the BAM Surplus Notes. For the year ended December 31, 2012, BAM had a pre-tax loss of $36.3 million that was recorded in net loss attributable to non-controlling interests, which included $18.4 million of interest expense on the BAM Surplus Notes. Sale of OneBeacon Runoff Business On October 17, 2012, one of OneBeacon’s indirect wholly-owned subsidiaries, OneBeacon Insurance Group LLC, entered into a definitive agreement with Trebuchet US Holdings, Inc. (“Trebuchet”), a wholly-owned subsidiary of Armour Group Holdings Limited (together with Trebuchet, “Armour”), to sell its runoff business (the “Runoff Transaction”). Pursuant to the terms of the agreement, at closing OneBeacon will transfer to Trebuchet all of the issued and outstanding shares of common stock of certain legal entities that will contain the assets, liabilities (including gross and ceded loss reserves) and capital supporting the runoff business as well as certain elements of the runoff business infrastructure, including staff and office space. The transaction is subject to regulatory approvals. For the year ended December 31, 2012, White Mountains recorded $91.5 million after-tax loss on sale and a $24.0 million in loss from operations, which included a $9.0 million after-tax loss related to an reduction in the workers compensation loss reserve discount rate on reserves being transferred as part of the sale (see Note 20). WM Solutions On December 3, 2012, WM Solutions acquired four runoff entities including Physicians Insurance Company of Ohio (“PICO”) and Citation Insurance Company (“Citation”) from PICO Holdings and two AIG runoff subsidiaries, American General Indemnity Company (“American General”) and American General Property Insurance Company (“American General Property”). The transactions resulted in a gain of $14.5 million recorded in other revenues. On December 30, 2011, WM Solutions acquired the runoff loss reserve portfolio of Old Lyme, a Bermuda-based reinsurer in runoff, for $6.0 million in cash and a purchase note for $2.1 million (see Note 6). The transaction resulted in a gain of $7.2 million recorded in other revenues. On February 26, 2010, WM Solutions acquired Central National for $5 million in cash. Central National ceased writing business in 1989 and has operated under the control of the Nebraska Department of Insurance since 1990. The transaction resulted in a gain of $12.8 million recorded in other revenues. Sale of AutoOne On August 30, 2011, OneBeacon entered into a definitive agreement (the “Purchase Agreement”) to sell the AutoOne business to Interboro Holdings, Inc. (“Interboro”). On February 22, 2012, OneBeacon completed the sale of AutoOne to Interboro Holdings, Inc. (“Interboro”). OneBeacon formed AutoOne in 2001 to provide products and services to automobile assigned risk markets primarily in New York and New Jersey. OneBeacon transferred to the buyer AutoOne Insurance Company (“AOIC”) and AutoOne Select Insurance Company (“AOSIC”), which contained the assets, liabilities (including loss reserves and unearned premiums), and the capital of the AutoOne business, and transferred substantially all of the AutoOne infrastructure including systems and office space as well as certain staff. As a result of the sale, AutoOne is reported as discontinued operations (see Note 20). Sale of Esurance On October 7, 2011, White Mountains completed the sale of Esurance Insurance and AFI to The Allstate Corporation (“Allstate”) for $700.0 million in excess of tangible book value. White Mountains recorded a gain on the sale of $677.5 million in discontinued operations. The transaction is subject to a true-up of the estimated tangible book value of the entities sold through the date of closing and certain other contingencies (see Note 19). OneBeacon Personal Lines and Commercial Lines Sales In July 2010, OneBeacon completed the sale of its traditional personal lines business (the “Personal Lines Transaction”) to Tower Group, Inc. (“Tower”). The Personal Lines Transaction included two insurance companies, York Insurance Company of Maine (“York”) and Massachusetts Homeland Insurance Company (“MHIC”), through which the majority of the traditional personal lines business was written on a direct basis. Subsequent to the transaction, OneBeacon cedes to Tower, on a 100% quota share basis, traditional personal lines business not directly written by York and MHIC; and OneBeacon assumes, on a 100% quota share basis, certain specialty lines business written directly by York. The Personal Lines Transaction also included two attorneys-in-fact managing the reciprocal insurance exchanges (“reciprocals”) that wrote the traditional personal lines business in New York and New Jersey, the surplus notes issued by the New York and New Jersey reciprocals, and the remaining renewal rights to certain other traditional personal lines insurance policies. The sale of the two attorneys-in-fact and the transfer of the surplus notes triggered deconsolidation of the reciprocals by White Mountains. OneBeacon and Tower also entered into a Transition Services Agreement (“TSA”), pursuant to which OneBeacon provides certain services to Tower during a three-year term. OneBeacon received $166.6 million as consideration, which was based upon the carrying value of the traditional personal lines business as of July 1, 2010. For the year ended December 31, 2010, OneBeacon recorded an after-tax net gain of $24.6 million on the sale that is comprised of $8.5 million included in other revenues and a $16.1 million tax benefit. OneBeacon’s second quarter financial statements included $5.6 million of the tax benefit, which related to the difference between the tax basis of the companies sold as part of the Personal Lines Transaction and the net asset value of those entities under GAAP. Net written premiums for the business sold were approximately $420.0 million for the year ended December 31, 2009. On December 3, 2009, OneBeacon sold the renewal rights to approximately $490.0 million in premiums from its non-specialty commercial lines business to The Hanover Insurance Group (“The Hanover”). The transaction includes small commercial accounts and the non-specialty portion of the middle-market business, beginning with January 1, 2010 effective dates (the “Commercial Lines Transaction”). As consideration for the Commercial Lines Transaction, OneBeacon received $23.2 million, reflected in other revenues. In accordance with the terms of the Commercial Lines Transaction, during the years ended December 31, 2011 and 2010, OneBeacon was paid additional consideration of $0.8 million and $10.2 million, reflected in other revenues, for aggregate premium renewals exceeding $200.0 million.

Significant Transactions

Formation of HG Global and BAM

In 2012, White Mountains capitalized HG Global with $594.5 million to fund BAM, a newly formed mutual municipal bond insurer. As of December 31, 2012, White Mountains owned 97.3% of HG Global’s preferred equity and 88.7% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM Surplus Notes. Through HG Re, which had statutory capital of $412.0 million at December 31, 2012, HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding, on a per policy basis. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time. For the year ended December 31, 2012, HG Global had pre-tax income of $14.2 million, which included $18.4 million of interest income on the BAM Surplus Notes. For the year ended December 31, 2012, BAM had a pre-tax loss of $36.3 million that was recorded in net loss attributable to non-controlling interests, which included $18.4 million of interest expense on the BAM Surplus Notes.

Sale of OneBeacon Runoff Business

On October 17, 2012, one of OneBeacon’s indirect wholly-owned subsidiaries, OneBeacon Insurance Group LLC, entered into a definitive agreement with Trebuchet US Holdings, Inc. (“Trebuchet”), a wholly-owned subsidiary of Armour Group Holdings Limited (together with Trebuchet, “Armour”), to sell its runoff business (the “Runoff Transaction”). Pursuant to the terms of the agreement, at closing OneBeacon will transfer to Trebuchet all of the issued and outstanding shares of common stock of certain legal entities that will contain the assets, liabilities (including gross and ceded loss reserves) and capital supporting the runoff business as well as certain elements of the runoff business infrastructure, including staff and office space. The transaction is subject to regulatory approvals.

For the year ended December 31, 2012, White Mountains recorded $91.5 million after-tax loss on sale and a $24.0 million in loss from operations, which included a $9.0 million after-tax loss related to an reduction in the workers compensation loss reserve discount rate on reserves being transferred as part of the sale (see Note 20).

WM Solutions

On December 3, 2012, WM Solutions acquired four runoff entities including Physicians Insurance Company of Ohio (“PICO”) and Citation Insurance Company (“Citation”) from PICO Holdings and two AIG runoff subsidiaries, American General Indemnity Company (“American General”) and American General Property Insurance Company (“American General Property”). The transactions resulted in a gain of $14.5 million recorded in other revenues.

On December 30, 2011, WM Solutions acquired the runoff loss reserve portfolio of Old Lyme, a Bermuda-based reinsurer in runoff, for $6.0 million in cash and a purchase note for $2.1 million (see Note 6). The transaction resulted in a gain of $7.2 million recorded in other revenues.

On February 26, 2010, WM Solutions acquired Central National for $5 million in cash. Central National ceased writing business in 1989 and has operated under the control of the Nebraska Department of Insurance since 1990. The transaction resulted in a gain of $12.8 million recorded in other revenues.

Sale of AutoOne

On August 30, 2011, OneBeacon entered into a definitive agreement (the “Purchase Agreement”) to sell the AutoOne business to Interboro Holdings, Inc. (“Interboro”). On February 22, 2012, OneBeacon completed the sale of AutoOne to Interboro Holdings, Inc. (“Interboro”). OneBeacon formed AutoOne in 2001 to provide products and services to automobile assigned risk markets primarily in New York and New Jersey. OneBeacon transferred to the buyer AutoOne Insurance Company (“AOIC”) and AutoOne Select Insurance Company (“AOSIC”), which contained the assets, liabilities (including loss reserves and unearned premiums), and the capital of the AutoOne business, and transferred substantially all of the AutoOne infrastructure including systems and office space as well as certain staff. As a result of the sale, AutoOne is reported as discontinued operations (see Note 20).

Sale of Esurance

On October 7, 2011, White Mountains completed the sale of Esurance Insurance and AFI to The Allstate Corporation (“Allstate”) for $700.0 million in excess of tangible book value. White Mountains recorded a gain on the sale of $677.5 million in discontinued operations. The transaction is subject to a true-up of the estimated tangible book value of the entities sold through the date of closing and certain other contingencies (see Note 19).

OneBeacon Personal Lines and Commercial Lines Sales

In July 2010, OneBeacon completed the sale of its traditional personal lines business (the “Personal Lines Transaction”) to Tower Group, Inc. (“Tower”). The Personal Lines Transaction included two insurance companies, York Insurance Company of Maine (“York”) and Massachusetts Homeland Insurance Company (“MHIC”), through which the majority of the traditional personal lines business was written on a direct basis. Subsequent to the transaction, OneBeacon cedes to Tower, on a 100% quota share basis, traditional personal lines business not directly written by York and MHIC; and OneBeacon assumes, on a 100% quota share basis, certain specialty lines business written directly by York. The Personal Lines Transaction also included two attorneys-in-fact managing the reciprocal insurance exchanges (“reciprocals”) that wrote the traditional personal lines business in New York and New Jersey, the surplus notes issued by the New York and New Jersey reciprocals, and the remaining renewal rights to certain other traditional personal lines insurance policies. The sale of the two attorneys-in-fact and the transfer of the surplus notes triggered deconsolidation of the reciprocals by White Mountains. OneBeacon and Tower also entered into a Transition Services Agreement (“TSA”), pursuant to which OneBeacon provides certain services to Tower during a three-year term.

OneBeacon received $166.6 million as consideration, which was based upon the carrying value of the traditional personal lines business as of July 1, 2010. For the year ended December 31, 2010, OneBeacon recorded an after-tax net gain of $24.6 million on the sale that is comprised of $8.5 million included in other revenues and a $16.1 million tax benefit. OneBeacon’s second quarter financial statements included $5.6 million of the tax benefit, which related to the difference between the tax basis of the companies sold as part of the Personal Lines Transaction and the net asset value of those entities under GAAP. Net written premiums for the business sold were approximately $420.0 million for the year ended December 31, 2009.

On December 3, 2009, OneBeacon sold the renewal rights to approximately $490.0 million in premiums from its non-specialty commercial lines business to The Hanover Insurance Group (“The Hanover”). The transaction includes small commercial accounts and the non-specialty portion of the middle-market business, beginning with January 1, 2010 effective dates (the “Commercial Lines Transaction”). As consideration for the Commercial Lines Transaction, OneBeacon received $23.2 million, reflected in other revenues. In accordance with the terms of the Commercial Lines Transaction, during the years ended December 31, 2011 and 2010, OneBeacon was paid additional consideration of $0.8 million and $10.2 million, reflected in other revenues, for aggregate premium renewals exceeding $200.0 million.