Summary of Significant Accounting Policies	9 Months Ended
Sep. 30, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation These interim consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively, with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance and reinsurance subsidiaries and affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. White Mountains’s reportable segments are OneBeacon, HG Global/BAM and Other Operations. As discussed further in Note 2 - “Significant Transactions”, on April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd., and its subsidiaries (collectively, “Sirius Group”) to CM International Holding PTE Ltd. (“CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. (See Note 2 - “Significant Transactions”). On June 9, 2016, White Mountains entered into an agreement to sell Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC, which was completed on July 21, 2016. For the three and nine months ended September 30, 2016, Sirius Group and Tranzact have been presented as discontinued operations in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation. The assets and liabilities of Sirius Group and Tranzact are classified as held for sale in the balance sheet as of December 31, 2015. (See Note 17 - “Held for Sale and Discontinued Operations”). The OneBeacon segment consists of OneBeacon Insurance Group, Ltd. (“OneBeacon Ltd.”), an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies (collectively, “OneBeacon”). OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products in the United States through independent agencies, regional and national brokers, wholesalers and managing general agencies. As of September 30, 2016 and December 31, 2015, White Mountains owned 76.1% and 75.5% of OneBeacon Ltd.’s outstanding common shares. The HG Global/BAM segment consists of HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is a municipal bond insurer domiciled in New York that was established in 2012 to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). HG Global, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), also provides 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. As of September 30, 2016 and December 31, 2015, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM, which is a mutual insurance company owned by its members. However, GAAP requires White Mountains to consolidate BAM’s results in its financial statements. BAM’s results are attributed to non-controlling interests. White Mountains’s Other Operations segment consists of the Company and its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), White Mountains’s variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts maturing by June 30, 2016, and its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”), and White Mountains’s ownership positions in QL Holdings, LLC (“MediaAlpha”) and Wobi Insurance Agency Ltd. (“Wobi”). On August 4, 2016, White Mountains acquired a 70.9% ownership share in Removal Stars Ltd (“Buzzmove”) for a purchase price of GBP 6.0 million (approximately $8.0 million based upon the foreign exchange spot rate at the date of acquisition), which is included in the Other Operations segment at September 30, 2016. The Other Operations segment also includes Abiaka Holdings, LLC, Star & Shield Services LLC, Star & Shield Risk Management LLC (“SSRM”), and Star & Shield Claims Services LLC (collectively “Star & Shield”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. White Mountains holds surplus notes issued by SSIE (the “SSIE Surplus Notes”) but does not have an ownership interest in SSIE. However, because SSRM’s role as the attorney-in-fact to SSIE gives it the power to direct the significant economic activities of SSIE, White Mountains is required to consolidate SSIE in its GAAP financial statements. SSIE’s results do not affect White Mountains’s common shareholders’ equity as they are attributable to non-controlling interests. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE surplus notes to National General Holdings Corp. As a result, White Mountains has presented Star & Shield's and SSIE's assets and liabilities as held for sale as of September 30, 2016 and December 31, 2015. All significant intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2015 Annual Report on Form 10-K. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Refer to the Company’s 2015 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. Recently Adopted Changes in Accounting Principles 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 September 30, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption. Fair Value Measurements On January 1, 2016, White Mountains adopted 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. Upon adoption, these fair value measurements are no longer classified within the fair value hierarchy. Prior year amounts have been modified to conform to the current year’s disclosures. 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 September 30, 2016, the unamortized debt issuance costs included in debt is $1.9 million. Recently Issued Accounting Pronouncements Short-Duration Contracts In May 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 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. 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). The new guidance 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. The updated guidance is 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, including reinsurance receivables, 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. 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. Stock Compensation In March 2016, the Board issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which is intended to simplify 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. The new guidance is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods with early adoption permitted. White Mountains does not expect a significant effect upon adoption. 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 guidance requires all equity securities with readily determinable fair values 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 investments without readily determinable fair values to identify impairment, and for impaired equity security investments 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 has taken the fair value election for its portfolio of equity security investments and, 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. 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 as well as commissions and other non-insurance revenues. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (ASC 606), which delayed the effective date of ASU 2014-09 to annual and interim reporting periods beginning after December 15, 2017. 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.

Summary of Significant Accounting Policies

Basis of Presentation

These interim consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively, with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance and reinsurance subsidiaries and affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. White Mountains’s reportable segments are OneBeacon, HG Global/BAM and Other Operations. As discussed further in Note 2 - “Significant Transactions”, on April 18, 2016, White Mountains completed its sale of Sirius International Insurance Group, Ltd., and its subsidiaries (collectively, “Sirius Group”) to CM International Holding PTE Ltd. (“CMI”), the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. (See Note 2 - “Significant Transactions”). On June 9, 2016, White Mountains entered into an agreement to sell Tranzact Holdings, LLC (“Tranzact”) to an affiliate of Clayton, Dubilier & Rice, LLC, which was completed on July 21, 2016. For the three and nine months ended September 30, 2016, Sirius Group and Tranzact have been presented as discontinued operations in the statement of operations and comprehensive income. Prior year amounts have been reclassified to conform to the current period’s presentation. The assets and liabilities of Sirius Group and Tranzact are classified as held for sale in the balance sheet as of December 31, 2015. (See Note 17 - “Held for Sale and Discontinued Operations”).

The OneBeacon segment consists of OneBeacon Insurance Group, Ltd. (“OneBeacon Ltd.”), an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies (collectively, “OneBeacon”). OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products in the United States through independent agencies, regional and national brokers, wholesalers and managing general agencies. As of September 30, 2016 and December 31, 2015, White Mountains owned 76.1% and 75.5% of OneBeacon Ltd.’s outstanding common shares.

The HG Global/BAM segment consists of HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is a municipal bond insurer domiciled in New York that was established in 2012 to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). HG Global, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), also provides 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. As of September 30, 2016 and December 31, 2015, White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM, which is a mutual insurance company owned by its members. However, GAAP requires White Mountains to consolidate BAM’s results in its financial statements. BAM’s results are attributed to non-controlling interests.

White Mountains’s Other Operations segment consists of the Company and its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), White Mountains’s variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which completed its runoff with all of its contracts maturing by June 30, 2016, and its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”), and White Mountains’s ownership positions in QL Holdings, LLC (“MediaAlpha”) and Wobi Insurance Agency Ltd. (“Wobi”). On August 4, 2016, White Mountains acquired a 70.9% ownership share in Removal Stars Ltd (“Buzzmove”) for a purchase price of GBP 6.0 million (approximately $8.0 million based upon the foreign exchange spot rate at the date of acquisition), which is included in the Other Operations segment at September 30, 2016. The Other Operations segment also includes Abiaka Holdings, LLC, Star & Shield Services LLC, Star & Shield Risk Management LLC (“SSRM”), and Star & Shield Claims Services LLC (collectively “Star & Shield”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. White Mountains holds surplus notes issued by SSIE (the “SSIE Surplus Notes”) but does not have an ownership interest in SSIE. However, because SSRM’s role as the attorney-in-fact to SSIE gives it the power to direct the significant economic activities of SSIE, White Mountains is required to consolidate SSIE in its GAAP financial statements. SSIE’s results do not affect White Mountains’s common shareholders’ equity as they are attributable to non-controlling interests. On August 19, 2016, White Mountains reached an agreement to sell Star & Shield and its investment in SSIE surplus notes to National General Holdings Corp. As a result, White Mountains has presented Star & Shield's and SSIE's assets and liabilities as held for sale as of September 30, 2016 and December 31, 2015.

All significant intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly state the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2015 Annual Report on Form 10-K.

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Refer to the Company’s 2015 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies.

Recently Adopted Changes in Accounting Principles

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 September 30, 2016 and accordingly, there was no effect to White Mountains’s financial statements upon adoption.

Fair Value Measurements

On January 1, 2016, White Mountains adopted 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. Upon adoption, these fair value measurements are no longer classified within the fair value hierarchy. Prior year amounts have been modified to conform to the current year’s disclosures.

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 September 30, 2016, the unamortized debt issuance costs included in debt is $1.9 million.

Recently Issued Accounting Pronouncements

Short-Duration Contracts

In May 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 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.

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). The new guidance 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. The updated guidance is 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, including reinsurance receivables, 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. 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.

Stock Compensation

In March 2016, the Board issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (ASC 718) which is intended to simplify 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. The new guidance is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods with early adoption permitted. White Mountains does not expect a significant effect upon adoption.

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 guidance requires all equity securities with readily determinable fair values 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 investments without readily determinable fair values to identify impairment, and for impaired equity security investments 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 has taken the fair value election for its portfolio of equity security investments and, 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. 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 as well as commissions and other non-insurance revenues. In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers (ASC 606), which delayed the effective date of ASU 2014-09 to annual and interim reporting periods beginning after December 15, 2017. 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.