Summary of Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2014
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Unrecognized Tax Benefits Effective January 1, 2014, White Mountains adopted ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss (“NOL”) carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryforward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance became effective for White Mountains on January 1, 2014. Adoption did not have any impact on White Mountains's financial condition, results of operations or cash flows. Recently Issued Accounting Pronouncements Revenue Recognition On May 28, 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606) which modifies the guidance for revenue recognition. The scope of the new ASU excludes insurance contracts but is applicable to certain fee arrangements, such as investment management fees. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows. ASU 2014-09 is effective for annual and interim reporting periods beginning after December 15, 2016. Share-Based Compensation Awards On June 19, 2014, the FASB issued ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period. The new guidance is intended to eliminate diversity in practice for employee share-based awards containing performance targets that could be achieved after the requisite service period. Some reporting entities account for performance targets that can be achieved after the requisite service period as performance conditions that affect the vesting of the award while other reporting entities treat those performance targets as nonvesting conditions that affect the grant-date fair value of the award. The updated guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost should be recognized in the period in which it becomes probable that the performance target will be achieved and should represent the compensation cost attributable to the periods for which service has been rendered. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2015. Qualified Affordable Housing Projects On January 15, 2014, the FASB issued ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323), which permits companies to make an accounting policy election to account for its investment in a QAHP using the proportional amortization method, if certain conditions are met. Under this method, the initial cost of the investment is amortized in proportion to the tax credits and other tax benefits received, with the net investment performance recognized in the income statement as a component of income tax expense. The new guidance also requires certain new disclosures for all QAHP investments. ASU 2014-01 is effective for annual and interim reporting periods beginning after December 15, 2014 and may be applied retrospectively to all periods presented upon adoption. White Mountains currently holds an investment in a QAHP that is accounted for under the equity method and does not expect the adoption to have a material impact on its financial position, results of operations or cash flows. Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity On April 10, 2014, the FASB issued ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity to reduce diversity in practice for reporting discontinued operations. Under the previous guidance, any component of an entity that was a reportable segment, an operating segment, a reporting unit, a subsidiary, or an asset group was eligible for discontinued operations presentation. The revised guidance only allows disposals of components of an entity that represent a strategic shift (e.g., disposal of a major geographical area, a major line of business, a major equity method investment, or other major parts of an entity) and that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. The revised guidance also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. The updated guidance is effective for transactions entered into after December 15, 2014.

Recently Adopted Changes in Accounting Principles

Unrecognized Tax Benefits

Effective January 1, 2014, White Mountains adopted ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss (“NOL”) carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryforward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance became effective for White Mountains on January 1, 2014. Adoption did not have any impact on White Mountains's financial condition, results of operations or cash flows.

Recently Issued Accounting Pronouncements

Revenue Recognition

On May 28, 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606) which modifies the guidance for revenue recognition. The scope of the new ASU excludes insurance contracts but is applicable to certain fee arrangements, such as investment management fees. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows. ASU 2014-09 is effective for annual and interim reporting periods beginning after December 15, 2016.

Share-Based Compensation Awards

On June 19, 2014, the FASB issued ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period. The new guidance is intended to eliminate diversity in practice for employee share-based awards containing performance targets that could be achieved after the requisite service period. Some reporting entities account for performance targets that can be achieved after the requisite service period as performance conditions that affect the vesting of the award while other reporting entities treat those performance targets as nonvesting conditions that affect the grant-date fair value of the award. The updated guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost should be recognized in the period in which it becomes probable that the performance target will be achieved and should represent the compensation cost attributable to the periods for which service has been rendered. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2015.

Qualified Affordable Housing Projects

On January 15, 2014, the FASB issued ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323), which permits companies to make an accounting policy election to account for its investment in a QAHP using the proportional amortization method, if certain conditions are met. Under this method, the initial cost of the investment is amortized in proportion to the tax credits and other tax benefits received, with the net investment performance recognized in the income statement as a component of income tax expense. The new guidance also requires certain new disclosures for all QAHP investments. ASU 2014-01 is effective for annual and interim reporting periods beginning after December 15, 2014 and may be applied retrospectively to all periods presented upon adoption. White Mountains currently holds an investment in a QAHP that is accounted for under the equity method and does not expect the adoption to have a material impact on its financial position, results of operations or cash flows.

Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity

On April 10, 2014, the FASB issued ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity to reduce diversity in practice for reporting discontinued operations. Under the previous guidance, any component of an entity that was a reportable segment, an operating segment, a reporting unit, a subsidiary, or an asset group was eligible for discontinued operations presentation. The revised guidance only allows disposals of components of an entity that represent a strategic shift (e.g., disposal of a major geographical area, a major line of business, a major equity method investment, or other major parts of an entity) and that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. The revised guidance also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. The updated guidance is effective for transactions entered into after December 15, 2014.