Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2017
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Stock Compensation Effective January 1, 2017, White Mountains adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which simplifies certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption White Mountains reported $6.5 million and $5.8 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the three-month periods ended March 31, 2017 and 2016. Such payments were classified as operating cash flows prior to adoption. In addition, the new guidance changed the treatment for excess tax benefits which arise from the difference between the deduction for tax purposes and the compensation costs recognized for financial reporting. Under the new guidance, a reporting entity will recognize excess tax benefits or expense in current period earnings, regardless of whether it is in a taxes payable position. Short-Duration Contracts Effective December 31, 2016, White Mountains adopted ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Upon adoption, White Mountains modified its footnote disclosures to include loss development tables on a disaggregated basis by accident year and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet. The footnotes disclosures have also been expanded to include information about claim frequency data, including a description of how the claims frequency data is measured. Prior year disclosures have been modified to conform to the new disclosures. See Note 3 — “Reserves for Unpaid Losses and Loss Adjustment Expenses”. Business Combinations - Measurement Period Adjustments Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments, which requires adjustments to provisional amounts recorded in connection with a business combination that are identified during the measurement period to be recorded in the reporting period in which the adjustment amounts are determined, rather than as retroactive adjustments to prior periods. White Mountains has not recognized any adjustments to estimated purchase accounting amounts for the year to date period ended March 31, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption. Amendments to Consolidation Analysis On January 1, 2016, White Mountains adopted 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. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments. Share-Based Compensation Awards On January 1, 2016, White Mountains adopted 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 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 is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures. Debt Issuance Costs On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of March 31, 2017, the unamortized debt issuance costs included in debt is $1.8 million. Recently Issued Accounting Pronouncements Cash Flow Statement In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230). Under current guidance, restricted amounts of cash or cash equivalents are excluded from the cash flow statement. The new guidance requires restricted cash and restricted cash equivalents to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows. In addition, the new guidance requires a description of the nature of the changes in restricted cash and cash equivalents during the periods presented. The updated guidance in ASU 2016-15 and ASU 2016-18 are both effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains is evaluating the expected impact of this new guidance. Credit Losses In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU, which applies to financial assets that have the contractual right to receive cash requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. The types of assets included in the scope of the new guidance includes premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains is evaluating the expected impact of this new guidance. Leases In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this new guidance and available adoption methods. Financial Instruments - Recognition and Measurement In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings accordingly, does not expect the adoption of ASU 2016-01 to have a significant impact on its financial statements. Revenue Recognition In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is to be recognized at an amount that reflects the consideration to which an entity expects to be entitled in exchange for goods or services transferred to customers. The new guidance sets forth the steps to be followed to recognize revenue: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. Subsequently, the FASB issued additional ASUs clarifying the guidance in and providing implementation guidance for ASU 2014-09. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers, which delays the effective date of ASU 2014-09 and all related ASUs to annual and interim reporting periods beginning after December 15, 2017. Most of White Mountains’s revenue from customers relates to insurance contracts, which are excluded from the scope of ASU 2014-09, as are investment income and investments gains and losses. However, the new guidance is applicable to some of White Mountains’s revenue streams, including certain fee arrangements as well as commissions and other non-insurance revenues. White Mountains is evaluating the new guidance, but does not expect ASU 2014-09 to have a significant effect on recognition of White Mountains’s non-insurance revenues from customers.

Recently Adopted Changes in Accounting Principles

Stock Compensation

Effective January 1, 2017, White Mountains adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which simplifies certain aspects of the accounting for share-based compensation. The new guidance provides an accounting policy election to account for forfeitures by either applying an assumption, as required under existing guidance, or by recognizing forfeitures when they actually occur. At adoption, White Mountains did not change its accounting policy for forfeitures, which is to apply an assumed forfeiture rate. The new guidance has also changed the threshold for partial cash settlement to settle statutory withholding requirements for equity classified awards, increasing the threshold up to the maximum statutory tax rate. As a result of adoption White Mountains reported $6.5 million and $5.8 million of statutory withholding tax payments made in connection with the settlement of restricted shares as financing cash flows for the three-month periods ended March 31, 2017 and 2016. Such payments were classified as operating cash flows prior to adoption.

In addition, the new guidance changed the treatment for excess tax benefits which arise from the difference between the deduction for tax purposes and the compensation costs recognized for financial reporting. Under the new guidance, a reporting entity will recognize excess tax benefits or expense in current period earnings, regardless of whether it is in a taxes payable position.

Short-Duration Contracts

Effective December 31, 2016, White Mountains adopted ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944), which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Upon adoption, White Mountains modified its footnote disclosures to include loss development tables on a disaggregated basis by accident year and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet. The footnotes disclosures have also been expanded to include information about claim frequency data, including a description of how the claims frequency data is measured. Prior year disclosures have been modified to conform to the new disclosures. See Note 3 — “Reserves for Unpaid Losses and Loss Adjustment Expenses”.

Business Combinations - Measurement Period Adjustments

Effective January 1, 2016, White Mountains adopted ASU 2015-16, Simplifying the Accounting for Measurement-Period Adjustments, which requires adjustments to provisional amounts recorded in connection with a business combination that are identified during the measurement period to be recorded in the reporting period in which the adjustment amounts are determined, rather than as retroactive adjustments to prior periods. White Mountains has not recognized any adjustments to estimated purchase accounting amounts for the year to date period ended March 31, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption.

Amendments to Consolidation Analysis

On January 1, 2016, White Mountains adopted 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. Adoption of ASU 2015-02 did not affect the consolidation analysis for any of White Mountains’s investments.

Share-Based Compensation Awards

On January 1, 2016, White Mountains adopted 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 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 is to be recognized in the period when it becomes probable the performance target will be achieved in an amount equal to the compensation cost attributable to the periods for which service has been rendered. Adoption did not have a significant effect on White Mountains’s financial position, results of operations, cash flows, presentation or disclosures.

Debt Issuance Costs

On January 1, 2016, White Mountains adopted ASU 2015-03, Imputation of Interest (ASC 835), which requires debt issuance costs to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. The new guidance requires amortization of debt issuance costs to be classified within interest expense and also requires disclosure to the debt’s effective interest rate. White Mountains has applied the guidance retrospectively and as a result has reclassified $1.9 million of unamortized debt issuance costs from other assets to debt as of December 31, 2015, reflecting these amounts as a reduction from the related debt, and has modified its disclosures to include the required effective interest rate on its debt. As of March 31, 2017, the unamortized debt issuance costs included in debt is $1.8 million.

Recently Issued Accounting Pronouncements

Cash Flow Statement

In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments (ASC 230), which addresses the classification and presentation of certain items, including debt prepayment and extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees, for which there was diversity in practice.

In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows: Restricted Cash (ASC 230). Under current guidance, restricted amounts of cash or cash equivalents are excluded from the cash flow statement. The new guidance requires restricted cash and restricted cash equivalents to be included in the reconciliation of beginning and end-of-period amounts presented on the statement of cash flows. In addition, the new guidance requires a description of the nature of the changes in restricted cash and cash equivalents during the periods presented.

The updated guidance in ASU 2016-15 and ASU 2016-18 are both effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. White Mountains is evaluating the expected impact of this new guidance.

Credit Losses

In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments (ASC 326), which establishes new guidance for the recognition of credit losses for financial assets measured at amortized cost. The new ASU, which applies to financial assets that have the contractual right to receive cash requires reporting entities to estimate the credit losses expected over the life of a credit exposure using historical information, current information and reasonable and supportable forecasts that affect the collectability of the financial asset. The types of assets included in the scope of the new guidance includes premium receivables, reinsurance recoverables and loans. ASU 2016-13 is effective for annual periods beginning after January 1, 2020, including interim periods. White Mountains is evaluating the expected impact of this new guidance.

Leases

In February 2016, the FASB issued ASU 2016-02, Leases (ASC 842). The new guidance requires lessees to recognize lease assets and liabilities on the balance sheet for both operating and financing leases, with the exception of leases with an original term of 12 months or less. Under existing guidance recognition of lease assets and liabilities is not required for operating leases. The lease assets and liabilities to be recognized are both measured initially based on the present value of the lease payments. Under the new guidance, a sale-leaseback transaction must meet the recognition criteria under ASC 606, Revenues in order to be accounted for as sale. The new guidance is effective for White Mountains for years beginning after December 15, 2018, including interim periods therein. White Mountains is evaluating the expected impact of this new guidance and available adoption methods.

Financial Instruments - Recognition and Measurement

In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities (ASC 825-10). The new ASU modifies the guidance for financial instruments, including investments in equity securities. Under the new guidance, all equity securities with readily determinable fair values are required to be measured at fair value with changes therein recognized through current period earnings. In addition, the new ASU requires a qualitative assessment for equity securities without readily determinable fair values to identify impairment, and for impaired equity securities to be measured at fair value. ASU 2016-01 is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings accordingly, does not expect the adoption of ASU 2016-01 to have a significant impact on its financial statements.

Revenue Recognition

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606), which modifies the guidance for revenue recognition. Under ASU 2014-09, revenue is to be recognized at an amount that reflects the consideration to which an entity expects to be entitled in exchange for goods or services transferred to customers. The new guidance sets forth the steps to be followed to recognize revenue: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. Subsequently, the FASB issued additional ASUs clarifying the guidance in and providing implementation guidance for ASU 2014-09. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers, which delays the effective date of ASU 2014-09 and all related ASUs to annual and interim reporting periods beginning after December 15, 2017. Most of White Mountains’s revenue from customers relates to insurance contracts, which are excluded from the scope of ASU 2014-09, as are investment income and investments gains and losses. However, the new guidance is applicable to some of White Mountains’s revenue streams, including certain fee arrangements as well as commissions and other non-insurance revenues. White Mountains is evaluating the new guidance, but does not expect ASU 2014-09 to have a significant effect on recognition of White Mountains’s non-insurance revenues from customers.