Summary of Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2015
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles 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 (ASC 205 and ASC 360) to reduce diversity in practice for reporting discontinued operations. ASU 2014-08 limits discontinued operations treatment to disposals 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. As discussed further in Note 2, White Mountains has entered into an agreement to sell Sirius Group, which has been classified as discontinued operations in accordance with ASU 2014-08. Qualified Affordable Housing Projects Effective January 1, 2015, White Mountains adopted ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323). ASU 2014-01 allows investors in QAHP to make a policy election to use the proportional amortization method. Under the proportional amortization method, the investor amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the investment results, net of the related tax benefits, as a component of income tax expense. Prior to adoption, White Mountains accounted for its QAHP investment under the equity method and recognized its share of its QAHP investment's losses in investment income. White Mountains made the policy election to account for its investment in its QAHP investment using the proportional amortization method, applied retrospectively. Under the proportional amortization method, the cumulative loss recognized through December 31, 2014 and December 31, 2013 was $0.9 million and $0.5 million. The retrospective adoption resulted in an increase of $0.4 million and $1.3 million to net investment income and a net increase of $0.6 million and $1.8 million to income tax expense for the three and nine months ended September 30, 2014. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above. Pushdown Accounting ASU 2014-17, Pushdown Accounting, a consensus of the FASB Emerging Issues Task Force (ASC 805) became effective upon its issuance on November 18, 2014. The new guidance, which is applicable prospectively, gives an acquired non-public company the option to apply pushdown accounting in its separate company financial statements in the period in which it is acquired in a change of control transaction. Once pushdown accounting has been applied, the election is irreversible. Acquired entities that chose not to apply pushdown accounting at the time of acquisition, may apply pushdown accounting in a subsequent period as a change in accounting principle under ASC 250, Accounting Changes and Error Corrections. White Mountains has not had any acquisitions for which it has elected to apply pushdown accounting since ASU 2014-17 became effective. 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 or financial statement presentation. Recently Issued Accounting Pronouncements Business Combinations - Measurement Period Adjustments On September 25, 2015, the FASB issued ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments. At the date of an acquisition, fair value of certain assets and liabilities may not be accurately determinable and are therefore recognized at the acquirer's best estimate. Such amounts may be updated as additional information becomes available in periods subsequent to the acquisition for up to one year. Prior to the issuance of this new ASU, subsequent adjustments had to be pushed back to the acquisition date, which required retroactive adjustments to prior period amounts. Under the new guidance, adjustments to provisional amounts that are identified during the measurement period are to be recorded in the reporting period in which the adjustment amounts are determined. ASU 2015-16 is effective for fiscal years beginning after December 15, 2015 and is applied prospectively to adjustments to estimated purchase accounting amounts that occur after the effective date. Early application is permitted. White Mountains has not recognized any adjustments to estimated purchase accounting amounts for the year to date period ended September 30, 2015. Short-Duration Contracts On May 21, 2015, the FASB issued ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944) which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Under the new guidance, some disclosures currently presented outside of White Mountains’s financial statements, such as loss development tables and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet, will become part of the financial statement footnotes. In addition, the loss development tables required to be presented under the new ASU must be presented on a disaggregated basis by accident year rather than by reporting year as currently presented. Some of the expanded disclosures are new requirements, such as the disclosure of reserves for losses incurred but not reported (“IBNR”) plus expected development on reported claims, which must be presented by accident year on a disaggregated basis. The new guidance also requires new disclosures about claim frequency data together with descriptions of the approach used to measure that data. Qualitative descriptions of methodologies and assumptions used to develop IBNR estimates must be presented together with the disaggregated amounts of IBNR to which they relate, along with a discussion of any significant changes in methodology and assumptions and the related effect upon the loss reserves. The new guidance will be effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016 with retrospective restatement of prior periods required. White Mountains will modify its financial statement footnote disclosures to conform to the requirements of ASU 2015-09 upon adoption, including revisions to prior year’s disclosures. Fair Value Measurements On May 1, 2015, the FASB issued ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820) which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient and has classified those measurements within Level 3 of the fair value hierarchy. The new guidance is effective for fiscal years beginning after December 15, 2015 and interim periods within those fiscal years. Debt Issuance Costs On April 7, 2015, the FASB issued ASU 2015-03, Imputation of Interest (ASC 835) which requires debt issuance costs related to a recognized debt liability to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. On August 18, 2015, the FASB issued ASU 2015-15, Imputation of Interest (ASC 835) which addresses the presentation and subsequent measurement of debt issuance costs related to line-of-credit arrangements. ASU 2015-03 and ASU 2-15-15 are effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect ASU 2015-03 or ASU 2015-15 to impact its financial position, results of operations, cash flows, presentation and disclosures. Amendments to Consolidation Analysis On February 18, 2015, the FASB issued ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. ASU 2015-02 is effective for annual and interim reporting periods beginning after December 15, 2015 and must be applied retrospectively. White Mountains does not expect ASU 2015-02 to affect the consolidation analysis for any of its existing investments. 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 (ASC 718). 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 non-vesting 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 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. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect adoption to have a significant effect on its financial position, results of operations, cash flows, presentation or disclosures. 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 third-party investment management fees charged by White Mountains Advisors, which were $2.0 million and $5.9 million for the three and nine months ended September 30, 2015 and $3.2 million and $9.2 million for the three and nine months ended September 30, 2014. 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. On August 12, 2015, the FASB issued ASU 2015-14, which defers the effective date of ASU 2014-09 to annual and interim reporting periods beginning after December 15, 2017.

Recently Adopted Changes in Accounting Principles

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 (ASC 205 and ASC 360) to reduce diversity in practice for reporting discontinued operations. ASU 2014-08 limits discontinued operations treatment to disposals 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.

As discussed further in Note 2, White Mountains has entered into an agreement to sell Sirius Group, which has been classified as discontinued operations in accordance with ASU 2014-08.

Qualified Affordable Housing Projects

Effective January 1, 2015, White Mountains adopted ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323). ASU 2014-01 allows investors in QAHP to make a policy election to use the proportional amortization method. Under the proportional amortization method, the investor amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the investment results, net of the related tax benefits, as a component of income tax expense. Prior to adoption, White Mountains accounted for its QAHP investment under the equity method and recognized its share of its QAHP investment's losses in investment income. White Mountains made the policy election to account for its investment in its QAHP investment using the proportional amortization method, applied retrospectively. Under the proportional amortization method, the cumulative loss recognized through December 31, 2014 and December 31, 2013 was $0.9 million and $0.5 million. The retrospective adoption resulted in an increase of $0.4 million and $1.3 million to net investment income and a net increase of $0.6 million and $1.8 million to income tax expense for the three and nine months ended September 30, 2014. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above.

Pushdown Accounting

ASU 2014-17, Pushdown Accounting, a consensus of the FASB Emerging Issues Task Force (ASC 805) became effective upon its issuance on November 18, 2014. The new guidance, which is applicable prospectively, gives an acquired non-public company the option to apply pushdown accounting in its separate company financial statements in the period in which it is acquired in a change of control transaction. Once pushdown accounting has been applied, the election is irreversible. Acquired entities that chose not to apply pushdown accounting at the time of acquisition, may apply pushdown accounting in a subsequent period as a change in accounting principle under ASC 250, Accounting Changes and Error Corrections. White Mountains has not had any acquisitions for which it has elected to apply pushdown accounting since ASU 2014-17 became effective.

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 or financial statement presentation.

Recently Issued Accounting Pronouncements

Business Combinations - Measurement Period Adjustments

On September 25, 2015, the FASB issued ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments. At the date of an acquisition, fair value of certain assets and liabilities may not be accurately determinable and are therefore recognized at the acquirer's best estimate. Such amounts may be updated as additional information becomes available in periods subsequent to the acquisition for up to one year. Prior to the issuance of this new ASU, subsequent adjustments had to be pushed back to the acquisition date, which required retroactive adjustments to prior period amounts. Under the new guidance, adjustments to provisional amounts that are identified during the measurement period are to be recorded in the reporting period in which the adjustment amounts are determined. ASU 2015-16 is effective for fiscal years beginning after December 15, 2015 and is applied prospectively to adjustments to estimated purchase accounting amounts that occur after the effective date. Early application is permitted. White Mountains has not recognized any adjustments to estimated purchase accounting amounts for the year to date period ended September 30, 2015.

Short-Duration Contracts

On May 21, 2015, the FASB issued ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944) which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Under the new guidance, some disclosures currently presented outside of White Mountains’s financial statements, such as loss development tables and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet, will become part of the financial statement footnotes. In addition, the loss development tables required to be presented under the new ASU must be presented on a disaggregated basis by accident year rather than by reporting year as currently presented. Some of the expanded disclosures are new requirements, such as the disclosure of reserves for losses incurred but not reported (“IBNR”) plus expected development on reported claims, which must be presented by accident year on a disaggregated basis. The new guidance also requires new disclosures about claim frequency data together with descriptions of the approach used to measure that data. Qualitative descriptions of methodologies and assumptions used to develop IBNR estimates must be presented together with the disaggregated amounts of IBNR to which they relate, along with a discussion of any significant changes in methodology and assumptions and the related effect upon the loss reserves. The new guidance will be effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016 with retrospective restatement of prior periods required. White Mountains will modify its financial statement footnote disclosures to conform to the requirements of ASU 2015-09 upon adoption, including revisions to prior year’s disclosures.

Fair Value Measurements

On May 1, 2015, the FASB issued ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820) which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient and has classified those measurements within Level 3 of the fair value hierarchy. The new guidance is effective for fiscal years beginning after December 15, 2015 and interim periods within those fiscal years.

Debt Issuance Costs

On April 7, 2015, the FASB issued ASU 2015-03, Imputation of Interest (ASC 835) which requires debt issuance costs related to a recognized debt liability to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. On August 18, 2015, the FASB issued ASU 2015-15, Imputation of Interest (ASC 835) which addresses the presentation and subsequent measurement of debt issuance costs related to line-of-credit arrangements. ASU 2015-03 and ASU 2-15-15 are effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect ASU 2015-03 or ASU 2015-15 to impact its financial position, results of operations, cash flows, presentation and disclosures.

Amendments to Consolidation Analysis

On February 18, 2015, the FASB issued ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. ASU 2015-02 is effective for annual and interim reporting periods beginning after December 15, 2015 and must be applied retrospectively. White Mountains does not expect ASU 2015-02 to affect the consolidation analysis for any of its existing investments.

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 (ASC 718). 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 non-vesting 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 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. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect adoption to have a significant effect on its financial position, results of operations, cash flows, presentation or disclosures.

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 third-party investment management fees charged by White Mountains Advisors, which were $2.0 million and $5.9 million for the three and nine months ended September 30, 2015 and $3.2 million and $9.2 million for the three and nine months ended September 30, 2014. 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. On August 12, 2015, the FASB issued ASU 2015-14, which defers the effective date of ASU 2014-09 to annual and interim reporting periods beginning after December 15, 2017.