Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2013
Accounting Policies [Abstract]
Noncontrolling Interests	Non-controlling Interests Non-controlling interests consist of the ownership interests of non-controlling shareholders in consolidated subsidiaries and are presented separately on the balance sheet. The percentage of the non-controlling shareholders' ownership interest in OneBeacon Ltd. at both March 31, 2013 and December 31, 2012 was 24.8%. 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 non-controlling 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 non-controlling 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, BAM's results do not affect White Mountains' common shareholders' equity as they are attributable to non-controlling interests. For the three months ended March 31, 2013, BAM reported $20.1 million in losses that have been allocated to non-controlling 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”), and received $245.7 million of proceeds, net of $4.3 million of issuance costs and commissions. SIG Preference Shares and dividends thereon are included in non-controlling interest on the balance sheet and on the statement of income and comprehensive income. At March 31, 2013 and December 31, 2012, the non-controlling equity interest in White Mountains' consolidated limited partnerships was $44.1 million and $41.5 million. At March 31, 2013 and December 31, 2012, the non-controlling equity interest in A.W.G. Dewar Inc, a subsidiary of OneBeacon, was $2.5 million and $2.8 million. At both March 31, 2013 and December 31, 2012, the non-controlling equity interest in Passage2Health Limited, a subsidiary of Sirius Group, was $0.2 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 boat business, no longer met the criteria for deferral. During the year ended December 31, 2012, White Mountains recognized $5.6 million of expense related to such previously deferrable acquisition costs that, if White Mountains had adopted ASU 2010-26 retrospectively, would have been recognized during 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).
Offsetting Assets and Liabilities	Offsetting Assets and Liabilities Effective January 1, 2013, White Mountains adopted ASU 2011-11, Disclosures about Offsetting Assets and Liabilities (ASC 210) and ASU 2013-01, Clarifying the Scope of Disclosures about Offsetting Assets and Liabilities. The new guidance expands the required disclosures in circumstances where either balances have been offset or the right of offset exists to make it easier for financial statement users to evaluate the effect or potential effect of netting arrangements on a reporting entity’s financial position. White Mountains is party to master netting arrangements in connection with derivative instruments held by WM Life Re and Sirius International. As a result of adoption, White Mountains has expanded its disclosures to present the gross amounts of assets and liabilities subject to master netting arrangements along with any related collateral amounts.