Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2012
Accounting Policies [Abstract]
Comprehensive Income	Comprehensive Income For fiscal periods beginning after December 15, 2011, ASU 2011-05, Comprehensive Income, became effective, which requires all components of comprehensive income to be reported in a continuous financial statement or in consecutive statements displaying the components of net income and the components of other comprehensive income. Since White Mountains previously presented comprehensive income in a continuous financial statement, adoption of ASU 2011-05 had no effect on White Mountains’ financial statement presentation.
Offsetting Assets and Liabilities	Offsetting Assets and Liabilities On December 16, 2011, the FASB issued ASU 2011-11, Disclosures about Offsetting Assets and Liabilities (ASC 210). The new standard expands the required disclosures in circumstances where either balances have been offset or the right of offset exists. The required disclosures are intended to provide information to enable financial statement users to evaluate the effect or potential effect of netting arrangements on a reporting entity’s financial position. Disclosures required under the new standard include the gross amount of assets and liabilities recognized; the amounts that have been offset to arrive at the amounts presented in the statement of financial position; and for any amounts subject to an enforceable master netting arrangement, whether such amounts have been offset. In addition, a description of the rights of offset should be disclosed. ASU 2011-11 is effective for periods beginning on or after January 1, 2013. White Mountains is party to master netting arrangements in connection with the derivative instruments held by WM Life Re and is currently evaluating the effect of adoption will have on its disclosures, but does not expect adoption to have a material effect on its financial position, results of operations or cash flows.
Noncontrolling Interest	Noncontrolling Interests Noncontrolling interests consist of the ownership interests of noncontrolling parties in consolidated entities and are presented separately as a component of equity on the balance sheet. The percentage of the noncontrolling equity interests in OneBeacon Ltd. at September 30, 2012 and December 31, 2011 was 24.8% and 24.5%. In July 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from certain management members of BAM, the latter of which is included in noncontrolling interest. Upon closing, certain BAM management members also received additional common and preferred shares of HG Global that resulted in a $2.2 million allocation of the carrying value of White Mountains' investment in HG Global to the noncontrolling interest, which was recorded as an adjustment to paid-in surplus in White Mountains' consolidated statement of changes in equity. White Mountains is required to consolidate BAM in its GAAP financial statements. However, since BAM is a mutual insurance company that is owned by its members and not White Mountains, BAM's results do not affect White Mountains' common shareholders' equity as they are attributable to noncontrolling interests. For the third quarter of 2012, BAM reported $18 million in pre-tax losses that have been allocated to noncontrolling interest. In May 2007, Sirius International Group, Ltd. (“SIG”), an intermediate holding company of Sirius Group, issued $250 million non-cumulative perpetual preference shares with a $1,000 per share liquidation preference (the “SIG Preference Shares”). Proceeds of $245.7 million, net of $4.3 million of issuance costs and commissions, were received from the issuance. The SIG Preference Shares are included in noncontrolling interests on the balance sheet. At September 30, 2012 and December 31, 2011, the noncontrolling equity interest in limited partnerships that are consolidated with White Mountains (the Prospector Offshore Fund, the Prospector Turtle Fund and Tuckerman Fund I prior to December 31, 2011) was $52.8 million and $54.2 million. On December 31, 2011, Tuckerman Fund I was dissolved and all of the net assets of the fund, which consisted of common stock of Hamer and Bri-Mar, were distributed. At September 30, 2012, the noncontrolling equity interest in Hamer and Bri-Mar was $4.5 million. At September 30, 2012 and December 31, 2011, the noncontrolling equity interest in A.W.G. Dewar Inc, a subsidiary of OneBeacon, was $3.2 million and $2.3 million. At September 30, 2012 and December 31, 2011, the noncontrolling equity interest in Passage2Health Limited, a subsidiary of Sirius Group, was $0.3 million and $0.6 million.
Policy Acquisition Costs	Policy Acquisition Costs On January 1, 2012, White Mountains adopted ASU 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts (ASC 944). The new standard changes the types of policy acquisition costs that are eligible for deferral. Specifically, the new guidance limits deferrable costs to those that are incremental direct costs of contract acquisition and certain costs related to acquisition activities performed by the insurer, such as underwriting, policy issuance and processing, medical and inspection costs and sales force contract selling. The ASU defines incremental direct costs as those costs that result directly from and were essential to the contract acquisition and would not have been incurred absent the acquisition. Accordingly, under the new guidance, deferrable acquisition costs are limited to costs related to successful contract acquisitions. Acquisition costs that are not eligible for deferral are to be charged to expense in the period incurred. White Mountains adopted ASU 2010-26 prospectively. Upon adoption, certain acquisition costs, primarily a portion of the profit sharing commissions associated with OneBeacon’s collector car and boats business, no longer meet the criteria for deferral. Deferred acquisition costs of $5.6 million at January 1, 2012 that no longer meet the criteria for deferral under ASU 2010-26 will be recognized in expense over the original amortization periods. For the three and nine months ended September 30, 2012, White Mountains recognized $0.9 million and $5.3 million of expense related to such previously deferrable acquisition costs. If White Mountains had adopted ASU 2010-26 retrospectively, $1.4 million and $5.7 million of acquisition costs that were deferred would have been recognized in expense for the three and nine months ended September 30, 2011.
Fair Value Measurements	Fair Value Measurements On January 1, 2012, White Mountains adopted ASU 2011-04, Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in U.S. GAAP and IFRS. The ASU clarifies existing guidance with respect to the concepts of highest and best use and valuation premise and measuring instruments classified within a reporting entity’s shareholders’ equity. The ASU also clarifies disclosure requirements, requiring disclosure of quantitative information about unobservable inputs used in Level 3 fair value measurements. The ASU also amends existing guidance. In circumstances where a reporting entity manages a portfolio of financial assets and liabilities based on the net market and counterparty credit risk exposures, the ASU permits determination of the fair value of those instruments to be based on the net risk exposure. In addition, the ASU permits the application of premiums or discounts to be applied in a fair value measurement to the extent that market participants would consider them in valuing the financial instruments. The ASU also expands the required disclosures for Level 3 measurements, requiring that reporting entities provide a narrative description of the sensitivity of Level 3 fair value measurements to changes in unobservable inputs and the interrelationships between those inputs, if any. As a result of adopting ASU 2011-04, White Mountains expanded its fair value disclosures. (See Note 5).