Basis of Presentation and Significant Accouting Policies (Policies)	6 Months Ended
Jun. 30, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 23 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The Company’s website is located at www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report. The accompanying 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. Consolidation Principles Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE. Intercompany transactions have been eliminated in consolidation. 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 2019 Annual Report on Form 10-K.
Business Combinations	Business Combinations White Mountains accounts for purchases of businesses using the acquisition method, which requires the measurement of assets acquired, including goodwill and other intangible assets, and liabilities assumed, including contingent liabilities, at their estimated fair values as of the acquisition date. The acquisition date fair values represent management’s best estimates and are based upon established valuation techniques, reasonable assumptions and, where appropriate, valuations performed by independent third parties. In circumstances where additional information is required in order to determine the acquisition date fair value of balance sheet amounts, provisional amounts may be recorded as of the acquisition date and may be subject to subsequent adjustment throughout the measurement period, which is up to one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the date of acquisition. White Mountains accounts for purchases of other intangible assets that do not meet the definition of a business as asset acquisitions. Asset acquisitions are recognized at the amount of consideration paid, which is deemed to equal fair value.
Use of Estimates	Use of Estimates 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.
Reportable Segments	Reportable Segments White Mountains has determined its reportable segments based on the nature of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. As of June 30, 2020, White Mountains’s reportable segments were HG Global/BAM, NSM, Kudu and Other Operations. On February 26, 2019, MediaAlpha completed the sale of a significant minority stake to Insignia Capital Group in connection with a recapitalization and cash distribution to existing equityholders (the “MediaAlpha Transaction”). White Mountains deconsolidated MediaAlpha as a result of the MediaAlpha Transaction, and consequently it was no longer a reportable segment. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations through the date of the MediaAlpha Transaction. See Note 2 — “Significant Transactions” and Note 13 — “Segment Information”. The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM, consisting of $203.0 million of Series A Notes and $300.0 million of Series B Notes (the “BAM Surplus Notes”). As of June 30, 2020 and December 31, 2019, 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. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests. The NSM segment consists of NSM Insurance HoldCo, LLC and its subsidiaries (collectively, “NSM”). NSM is a full-service managing general underwriting agency (“MGU”) and program administrator for specialty property and casualty insurance. The company places insurance in niche sectors such as specialty transportation, real estate, social services and pet health insurance. On behalf of its insurance carrier partners, NSM manages all aspects of the placement process, including product development, marketing, underwriting, policy issuance and claims. NSM earns commissions based on the volume and profitability of the insurance that it places. NSM does not take insurance risk. As of June 30, 2020 and December 31, 2019, White Mountains owned 96.6% and 96.4% of the basic units outstanding of NSM (89.6% and 88.4% on a fully diluted, fully converted basis). See Note 2 — “Significant Transactions”. The Kudu segment consists of Kudu Investment Management, LLC and its subsidiaries (collectively “Kudu”), a capital solutions provider for asset management firms. Kudu provides capital solutions for boutique asset managers for a variety of purposes including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu also provides strategic assistance to investees from time to time. Kudu’s capital solutions typically are structured as minority preferred equity stakes with distribution rights, typically tied to gross revenues and designed to generate immediate strong, stable cash yields. On April 4, 2019, White Mountains acquired the ownership interests in Kudu held by certain funds managed by Oaktree Capital Management, L.P. (“Oaktree”) for cash consideration of $81.4 million. In addition, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total capital commitment to $250.0 million (the “Kudu Transaction”). As a result of the Kudu Transaction, White Mountains’s basic unit ownership of Kudu increased from 49.5% to 99.1% (42.7% to 85.4% on a fully diluted, fully converted basis), and White Mountains began consolidating Kudu in its financial statements during the second quarter of 2019. During the fourth quarter of 2019, White Mountains increased its total capital commitment to Kudu by an additional $100.0 million to $350.0 million and, Kudu obtained a committed $125.0 million credit facility. See Note 2 — “Significant Transactions”. The Other Operations segment consists of the Company and its wholly-owned subsidiary, White Mountains Capital, Inc., its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha (for periods after the MediaAlpha Transaction), PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/DavidShield”), Elementum Holdings LP (“Elementum”) and Kudu (for periods prior to the Kudu Transaction), and certain other consolidated and unconsolidated entities and certain other strategic investments. The MediaAlpha segment consisted of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a marketing technology company. It operates a transparent and efficient online customer acquisition technology platform that facilitates real-time transactions between buyers and sellers of consumer referrals (e.g., clicks, calls and leads), primarily in the property & casualty, health and life insurance verticals.
Discontinued Operations and Assets and Liabilities Held for Sale	Discontinued Operations and Assets Held for Sale White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of June 30, 2020 and December 31, 2019.
Reinsurance Contracts Accounted For As Deposits	Reinsurance Contracts Accounted for as Deposits Reinsurance contracts that do not meet the risk transfer requirements necessary to be accounted for as reinsurance are accounted for using the deposit method. Under the deposit method, ceded premiums paid are not recognized through income but rather treated as a deposit. BAM entered into a ceded reinsurance contract agreement with Fidus Reinsurance Ltd. (“Fidus Re”) during the quarter ended June 30, 2018, which is accounted for using the deposit method. See Note 8 — “Municipal Bond Guarantee Insurance”. The nonrefundable consideration paid by BAM to Fidus Re is charged to financing expense within general and administrative expenses.
Restricted Cash	Restricted Cash Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated subsidiaries. Cash balances that are not immediately available for general corporate purposes, including fiduciary accounts held by NSM on behalf of insurance carriers, are classified as restricted. Restricted amounts included within cash are disclosed parenthetically on the balance sheet. Changes in restricted cash balances are presented as a separate caption within the operations, investing and financing activities sections of the statement of cash flows.
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Income Taxes On January 1, 2020, White Mountains adopted ASU 2019-12, Simplifying the Accounting for Income Taxes (ASC 740), which removes exceptions to standard guidance. Under the new guidance non-income-based taxes, such as franchise taxes, are reported within pre-tax income rather than being included in income taxes. In addition, the new guidance eliminates the exception to the incremental approach for interperiod tax allocation, which previously allowed consideration of the tax effects of items such as discontinued operations and items recognized through other comprehensive income. For periods subsequent to the adoption of ASU 2019-12, White Mountains records both the tax expense related to BAM’s members surplus contributions (“MSC”) and the related valuation allowance on such taxes through the non-controlling interest equity. Prior to the adoption of ASU 2019-12, White Mountains recorded the tax expense related to BAM’s MSC directly to non-controlling interest equity, while the valuation allowance on such taxes was recorded through the income statement. Goodwill On January 1, 2020, White Mountains adopted ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changed the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same, but the second step of the two-step quantitative test, which required calculation of the implied fair value of goodwill, has been eliminated. Instead, an impairment charge is recognized when the carrying value of a reporting unit exceeds its fair value. Any excess of carrying value over fair value is written down as an impairment. White Mountains did not recognize an impairment of goodwill during the three and six months ended June 30, 2020 and accordingly, adoption of ASU 2017-04 had no impact on White Mountains’s financial statements. Credit Losses On January 1, 2020, White Mountains adopted 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 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. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, adoption of ASU 2016-13 had no impact on White Mountains’s financial statements Leases On January 1, 2019, White Mountains adopted ASU 2016-02, Leases (ASC 842), which 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. White Mountains elected the optional transition method that permits prospective adoption with recognition of a cumulative effect adjustment to the opening balance of retained earnings. As a result, White Mountains has presented comparative periods prior to adoption in accordance with previous lease accounting guidance. White Mountains also elected all available practical expedients permitted under ASC 842, which allowed White Mountains to carryforward its historical lease classification and not reassess leases for the definition of a lease under the new guidance. As of January 1, 2019, White Mountains recognized $23.2 million for both the lease right-of-use assets and lease liabilities. As of June 30, 2020, White Mountains recognized $34.0 million and $34.2 million of lease right-of-use assets and lease liabilities. Adoption of ASU 2016-02 did not result in an adjustment to opening retained earnings. Premium Amortization on Callable Debt Securities On January 1, 2019, White Mountains adopted ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium is amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. Adoption of ASU 2017-08 had no impact on White Mountains’s financial statements.

Basis of Presentation and Significant Accouting Policies (Policies)

6 Months Ended

Jun. 30, 2020

The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its subsidiaries and other affiliates. The Company’s headquarters is located at 26 Reid Street, Hamilton, Bermuda HM 11, its principal executive office is located at 23 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. The Company’s website is located at www.whitemountains.com. The information contained on White Mountains’s website is not incorporated by reference into, and is not a part of, this report.

The accompanying 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.

Consolidation Principles

Under GAAP, the Company is required to consolidate any entity in which it holds a controlling financial interest. A controlling financial interest is usually in the form of an investment representing the majority of the subsidiary’s voting interests. However, a controlling financial interest may also arise from a financial interest in a variable interest entity (“VIE”) through arrangements that do not involve ownership of voting interests. The Company consolidates a VIE if it determines that it is the primary beneficiary. The primary beneficiary is defined as the entity who holds a variable interest that gives it both the power to direct the VIE’s activities that most significantly impact its economic performance and the obligation to absorb losses of, or the right to receive returns from, the VIE that could potentially be significant to the VIE.

Intercompany transactions have been eliminated in consolidation. 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 2019 Annual Report on Form 10-K.

Business Combinations

White Mountains accounts for purchases of businesses using the acquisition method, which requires the measurement of assets acquired, including goodwill and other intangible assets, and liabilities assumed, including contingent liabilities, at their estimated fair values as of the acquisition date. The acquisition date fair values represent management’s best estimates and are based upon established valuation techniques, reasonable assumptions and, where appropriate, valuations performed by independent third parties. In circumstances where additional information is required in order to determine the acquisition date fair value of balance sheet amounts, provisional amounts may be recorded as of the acquisition date and may be subject to subsequent adjustment throughout the measurement period, which is up to one year from the acquisition date. Measurement period adjustments are recognized in the period in which they are determined. The results of operations and cash flows of businesses acquired are included in the consolidated financial statements from the date of acquisition. White Mountains accounts for purchases of other intangible assets that do not meet the definition of a business as asset acquisitions. Asset acquisitions are recognized at the amount of consideration paid, which is deemed to equal fair value.

Use of Estimates

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.

Reportable Segments

White Mountains has determined its reportable segments based on the nature of the underlying businesses, the manner in which the Company’s subsidiaries and affiliates are organized and managed and the organization of the financial information provided to the chief operating decision maker to assess performance and make decisions regarding allocation of resources. As of June 30, 2020, White Mountains’s reportable segments were HG Global/BAM, NSM, Kudu and Other Operations. On February 26, 2019, MediaAlpha completed the sale of a significant minority stake to Insignia Capital Group in connection with a recapitalization and cash distribution to existing equityholders (the “MediaAlpha Transaction”). White Mountains deconsolidated MediaAlpha as a result of the MediaAlpha Transaction, and consequently it was no longer a reportable segment. White Mountains’s consolidated statement of comprehensive income and its segment disclosures include MediaAlpha’s results of operations through the date of the MediaAlpha Transaction. See Note 2 — “Significant Transactions” and Note 13 — “Segment Information”.

The HG Global/BAM segment consists of HG Global Ltd. and its wholly-owned subsidiaries (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”) (collectively, “HG Global/BAM”). BAM is the first and only mutual municipal bond insurance company in the United States. By insuring the timely payment of principal and interest, BAM provides market access to, and lowers interest expense for, issuers of municipal bonds used to finance essential public purposes such as schools, utilities and transportation facilities. BAM is owned by and operated for the benefit of its members, the municipalities that purchase BAM’s insurance for their debt issuances. HG Global was established to fund the startup of BAM and, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), to provide up to 15%-of-par, first loss reinsurance protection for policies underwritten by BAM. HG Global, together with its subsidiaries, funded the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM, consisting of $203.0 million of Series A Notes and $300.0 million of Series B Notes (the “BAM Surplus Notes”). As of June 30, 2020 and December 31, 2019, 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. However, White Mountains is required to consolidate BAM’s results in its financial statements because BAM is a VIE for which White Mountains is the primary beneficiary. BAM’s results are attributed to non-controlling interests.

The NSM segment consists of NSM Insurance HoldCo, LLC and its subsidiaries (collectively, “NSM”). NSM is a full-service managing general underwriting agency (“MGU”) and program administrator for specialty property and casualty insurance. The company places insurance in niche sectors such as specialty transportation, real estate, social services and pet health insurance. On behalf of its insurance carrier partners, NSM manages all aspects of the placement process, including product development, marketing, underwriting, policy issuance and claims. NSM earns commissions based on the volume and profitability of the insurance that it places. NSM does not take insurance risk. As of June 30, 2020 and December 31, 2019, White Mountains owned 96.6% and 96.4% of the basic units outstanding of NSM (89.6% and 88.4% on a fully diluted, fully converted basis). See Note 2 — “Significant Transactions”.

The Kudu segment consists of Kudu Investment Management, LLC and its subsidiaries (collectively “Kudu”), a capital solutions provider for asset management firms. Kudu provides capital solutions for boutique asset managers for a variety of purposes including generational ownership transfers, management buyouts, acquisition and growth finance and legacy partner liquidity. Kudu also provides strategic assistance to investees from time to time. Kudu’s capital solutions typically are structured as minority preferred equity stakes with distribution rights, typically tied to gross revenues and designed to generate immediate strong, stable cash yields. On April 4, 2019, White Mountains acquired the ownership interests in Kudu held by certain funds managed by Oaktree Capital Management, L.P. (“Oaktree”) for cash consideration of $81.4 million. In addition, White Mountains assumed all of Oaktree’s unfunded capital commitments to Kudu, increasing White Mountains’s total capital commitment to $250.0 million (the “Kudu Transaction”). As a result of the Kudu Transaction, White Mountains’s basic unit ownership of Kudu increased from 49.5% to 99.1% (42.7% to 85.4% on a fully diluted, fully converted basis), and White Mountains began consolidating Kudu in its financial statements during the second quarter of 2019. During the fourth quarter of 2019, White Mountains increased its total capital commitment to Kudu by an additional $100.0 million to $350.0 million and, Kudu obtained a committed $125.0 million credit facility. See Note 2 — “Significant Transactions”.

The Other Operations segment consists of the Company and its wholly-owned subsidiary, White Mountains Capital, Inc., its other intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), investment assets managed by WM Advisors, its interests in MediaAlpha (for periods after the MediaAlpha Transaction), PassportCard Limited (“PassportCard”) and DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) (collectively, “PassportCard/DavidShield”), Elementum Holdings LP (“Elementum”) and Kudu (for periods prior to the Kudu Transaction), and certain other consolidated and unconsolidated entities and certain other strategic investments.

The MediaAlpha segment consisted of QL Holdings LLC and its wholly-owned subsidiary QuoteLab, LLC (collectively “MediaAlpha”). MediaAlpha is a marketing technology company. It operates a transparent and efficient online customer acquisition technology platform that facilitates real-time transactions between buyers and sellers of consumer referrals (e.g., clicks, calls and leads), primarily in the property & casualty, health and life insurance verticals.

Discontinued Operations and Assets and Liabilities Held for Sale

Discontinued Operations and Assets Held for Sale

White Mountains has classified its Guilford, Connecticut property, which consists of an office building and adjacent land, as held for sale as of June 30, 2020 and December 31, 2019.

Reinsurance Contracts Accounted For As Deposits

Reinsurance Contracts Accounted for as Deposits

Reinsurance contracts that do not meet the risk transfer requirements necessary to be accounted for as reinsurance are accounted for using the deposit method. Under the deposit method, ceded premiums paid are not recognized through income but rather treated as a deposit. BAM entered into a ceded reinsurance contract agreement with Fidus Reinsurance Ltd. (“Fidus Re”) during the quarter ended June 30, 2018, which is accounted for using the deposit method. See Note 8 — “Municipal Bond Guarantee Insurance”. The nonrefundable consideration paid by BAM to Fidus Re is charged to financing expense within general and administrative expenses.

Restricted Cash

Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the

statement of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company’s consolidated

subsidiaries.

Cash balances that are not immediately available for general corporate purposes, including fiduciary accounts held by NSM on behalf of insurance carriers, are classified as restricted. Restricted amounts included within cash are disclosed parenthetically on the balance sheet. Changes in restricted cash balances are presented as a separate caption within the operations, investing and financing activities sections of the statement of cash flows.

Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements

Recently Adopted Changes in Accounting Principles

Income Taxes

On January 1, 2020, White Mountains adopted ASU 2019-12, Simplifying the Accounting for Income Taxes (ASC 740), which removes exceptions to standard guidance. Under the new guidance non-income-based taxes, such as franchise taxes, are reported within pre-tax income rather than being included in income taxes. In addition, the new guidance eliminates the exception to the incremental approach for interperiod tax allocation, which previously allowed consideration of the tax effects of items such as discontinued operations and items recognized through other comprehensive income.

For periods subsequent to the adoption of ASU 2019-12, White Mountains records both the tax expense related to BAM’s members surplus contributions (“MSC”) and the related valuation allowance on such taxes through the non-controlling interest equity. Prior to the adoption of ASU 2019-12, White Mountains recorded the tax expense related to BAM’s MSC directly to non-controlling interest equity, while the valuation allowance on such taxes was recorded through the income statement.

Goodwill

On January 1, 2020, White Mountains adopted ASU 2017-04, Simplifying the Test for Goodwill Impairment (ASC 350), which changed the guidance on goodwill impairment testing. Under the new guidance, the qualitative assessment of the recoverability of goodwill remains the same, but the second step of the two-step quantitative test, which required calculation of the implied fair value of goodwill, has been eliminated. Instead, an impairment charge is recognized when the carrying value of a reporting unit exceeds its fair value. Any excess of carrying value over fair value is written down as an impairment. White Mountains did not recognize an impairment of goodwill during the three and six months ended June 30, 2020 and accordingly, adoption of ASU 2017-04 had no impact on White Mountains’s financial statements.

Credit Losses

On January 1, 2020, White Mountains adopted 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 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. White Mountains measures its portfolio of investment securities at fair value with changes therein recognized through current period earnings and, accordingly, adoption of ASU 2016-13 had no impact on White Mountains’s financial statements

Leases

On January 1, 2019, White Mountains adopted ASU 2016-02, Leases (ASC 842), which 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. White Mountains elected the optional transition method that permits prospective adoption with recognition of a cumulative effect adjustment to the opening balance of retained earnings. As a result, White Mountains has presented comparative periods prior to adoption in accordance with previous lease accounting guidance. White Mountains also elected all available practical expedients permitted under ASC 842, which allowed White Mountains to carryforward its historical lease classification and not reassess leases for the definition of a lease under the new guidance. As of January 1, 2019, White Mountains recognized $23.2 million for both the lease right-of-use assets and lease liabilities. As of June 30, 2020, White Mountains recognized $34.0 million and $34.2 million of lease right-of-use assets and lease liabilities. Adoption of ASU 2016-02 did not result in an adjustment to opening retained earnings.

Premium Amortization on Callable Debt Securities

On January 1, 2019, White Mountains adopted ASU 2017-08, Premium Amortization on Purchased Callable Debt Securities (ASC 310-20), which changes the amortization period for certain purchased callable debt securities. Under the new guidance, for investments in callable debt securities held at a premium, the premium is amortized over the period to the earliest call date. The new guidance does not change the amortization period for callable debt securities held at a discount. Adoption of ASU 2017-08 had no impact on White Mountains’s financial statements.