Summary of significant accounting policies	6 Months Ended
Jun. 30, 2020
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies Basis of presentation The accompanying Unaudited Consolidated Financial Statements at June 30, 2020, have been prepared in accordance with generally accepted accounting principles in the United States of America ("GAAP") and the rules and regulations of the U.S. Securities and Exchange Commission ("SEC") for interim financial information. The accompanying Unaudited Consolidated Financial Statements present the consolidated results of operations, financial condition, and cash flows of the Company and its subsidiaries and those entities in which the Company has control and a majority economic interest as well as those variable interest entities ("VIEs") that meet the requirements for consolidation. All significant intercompany transactions have been eliminated in consolidation. These Unaudited Consolidated Financial Statements do not include all disclosures normally included in annual financial statements prepared in accordance with GAAP and should be read in conjunction with the Audited Consolidated Financial Statements and the related notes for the year ended December 31, 2019. The consolidated financial information as of December 31, 2019 included herein has been derived from the Audited Consolidated Financial Statements as of December 31, 2019. Effective January 1, 2020, the Company changed its reportable segments. The change in reportable segments had no impact on the Company’s historical consolidated financial positions, results of operations or cash flows as previously reported. Where applicable, all prior periods presented have been revised to conform to this new presentation. 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. In the opinion of management, the accompanying Unaudited Consolidated Financial Statements reflect all adjustments (consisting of normally recurring accruals) necessary for a fair statement of results on an interim basis. Tabular dollar amounts are in millions, with the exception of share and per share amounts. All amounts are reported in U.S. dollars, except where noted otherwise. Recently adopted changes in accounting principles Credit losses Effective January 1, 2020, Sirius Group adopted Accounting Standards Update ("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 ("current expected credit losses" or "CECL"). The guidance applies to financial assets that have the contractual right to receive cash, including reinsurance receivables and recoverables and 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 expected credit losses, and subsequent adjustments to such losses, are recorded through an allowance account that is deducted from the amortized cost basis of the financial asset, with the net carrying value of the financial asset presented on the consolidated balance sheet at the amount expected to be collected. Sirius Group adopted the new guidance using a modified retrospective transition method through a cumulative-effect adjustment to retained earnings. As a result of the adoption of the new guidance, Sirius Group estimated a total CECL allowance of $14.9 million. The adoption of this guidance did not materially impact our results of operations or cash flows. Due to the adoption of the standard using the retrospective cumulative-effect adjustment method, the Company recorded $(6.8) million cumulative-effect adjustment net of taxes to its opening retained earnings. (See Note 7.) Fair Value Measurement Disclosures Effective January 1, 2020, Sirius Group adopted ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which modifies the disclosure requirements of fair value measurements. The adoption of this guidance did not have any impact on the consolidated financial statements but expanded disclosure of the ranges and weighted averages of significant unobservable inputs used in Level 3 fair values. (See Note 9.) Simplifying the Accounting for Income Taxes Effective January 1, 2020, Sirius Group adopted ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which eliminates certain exceptions, and clarifies and simplifies certain aspects of accounting for income taxes. Sirius Group has fully adopted all provisions of the guidance with consideration of the various transition methods. Sirius Group adopted the removal of the exception to the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss for the year. The removal of this exception allowed Sirius Group to record a benefit for a year-to-date loss in excess of its forecasted loss in certain jurisdictions. The removal of this exception was applied prospectively to the Income tax benefit (expense) line on the Consolidated Statements of (Loss) Income. Sirius Group adopted all other provisions in the guidance, including the requirement for an entity to recognize a franchise tax (or similar tax) that is partially based on income as an income-based tax and account for any incremental amount incurred as a non-income-based tax through a cumulative-effect adjustment to retained earnings. These provisions did not have a material impact on the Consolidated Financial Statements or were not applicable to Sirius Group. Significant accounting policies Long-Lived Assets Classified as Held for Sale A long-lived asset or a disposal group classified as held for sale (but not qualifying for presentation as a discontinued operation in the Consolidated Financial Statements) is presented separately in the Consolidated Balance Sheets and is measured at the lower of its carrying amount or fair value less cost to sell. The assets and liabilities of a disposal group classified as held for sale are presented separately in the asset and liability sections, respectively, of the Consolidated Balance Sheets and are not netted. The major classes of assets and liabilities classified as held for sale are separately disclosed in the notes to financial statements. (See Note 3.)

Summary of significant accounting policies

Basis of presentation

The accompanying Unaudited Consolidated Financial Statements at June 30, 2020, have been prepared in accordance with generally accepted accounting principles in the United States of America ("GAAP") and the rules and regulations of the U.S. Securities and Exchange Commission ("SEC") for interim financial information. The accompanying Unaudited Consolidated Financial Statements present the consolidated results of operations, financial condition, and cash flows of the Company and its subsidiaries and those entities in which the Company has control and a majority economic interest as well as those variable interest entities ("VIEs") that meet the requirements for consolidation. All significant intercompany transactions have been eliminated in consolidation.

These Unaudited Consolidated Financial Statements do not include all disclosures normally included in annual financial statements prepared in accordance with GAAP and should be read in conjunction with the Audited Consolidated Financial Statements and the related notes for the year ended December 31, 2019. The consolidated financial information as of December 31, 2019 included herein has been derived from the Audited Consolidated Financial Statements as of December 31, 2019.

Effective January 1, 2020, the Company changed its reportable segments. The change in reportable segments had no impact on the Company’s historical consolidated financial positions, results of operations or cash flows as previously reported. Where applicable, all prior periods presented have been revised to conform to this new presentation.

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. In the opinion of management, the accompanying Unaudited Consolidated Financial Statements reflect all adjustments (consisting of normally recurring accruals) necessary for a fair statement of results on an interim basis. Tabular dollar amounts are in millions, with the exception of share and per share amounts. All amounts are reported in U.S. dollars, except where noted otherwise.

Recently adopted changes in accounting principles

Credit losses

Effective January 1, 2020, Sirius Group adopted Accounting Standards Update ("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 ("current expected credit losses" or "CECL"). The guidance applies to financial assets that have the contractual right to receive cash, including reinsurance receivables and recoverables and 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 expected credit losses, and subsequent adjustments to such losses, are recorded through an allowance account that is deducted from the amortized cost basis of the financial asset, with the net carrying value of the financial asset presented on the consolidated balance sheet at the amount expected to be collected. Sirius Group adopted the new guidance using a modified retrospective transition method through a cumulative-effect adjustment to retained earnings.

As a result of the adoption of the new guidance, Sirius Group estimated a total CECL allowance of $14.9 million. The adoption of this guidance did not materially impact our results of operations or cash flows. Due to the adoption of the standard using the retrospective cumulative-effect adjustment method, the Company recorded $(6.8) million cumulative-effect adjustment net of taxes to its opening retained earnings. (See Note 7.)

Fair Value Measurement Disclosures

Effective January 1, 2020, Sirius Group adopted ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which modifies the disclosure requirements of fair value measurements. The adoption of this guidance did not have any impact on the consolidated financial statements but expanded disclosure of the ranges and weighted averages of significant unobservable inputs used in Level 3 fair values. (See Note 9.)

Simplifying the Accounting for Income Taxes

Effective January 1, 2020, Sirius Group adopted ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which eliminates certain exceptions, and clarifies and simplifies certain aspects of accounting for income taxes. Sirius Group has fully adopted all provisions of the guidance with consideration of the various transition methods.

Sirius Group adopted the removal of the exception to the general methodology for calculating income taxes in an interim period when a year-to-date loss exceeds the anticipated loss for the year. The removal of this exception allowed Sirius Group to record a benefit for a year-to-date loss in excess of its forecasted loss in certain jurisdictions. The removal of this exception was applied prospectively to the Income tax benefit (expense) line on the Consolidated Statements of (Loss) Income.

Sirius Group adopted all other provisions in the guidance, including the requirement for an entity to recognize a franchise tax (or similar tax) that is partially based on income as an income-based tax and account for any incremental amount incurred as a non-income-based tax through a cumulative-effect adjustment to retained earnings. These provisions did not have a material impact on the Consolidated Financial Statements or were not applicable to Sirius Group.

Significant accounting policies

Long-Lived Assets Classified as Held for Sale

A long-lived asset or a disposal group classified as held for sale (but not qualifying for presentation as a discontinued operation in the Consolidated Financial Statements) is presented separately in the Consolidated Balance Sheets and is measured at the lower of its carrying amount or fair value less cost to sell. The assets and liabilities of a disposal group classified as held for sale are presented separately in the asset and liability sections, respectively, of the Consolidated Balance Sheets and are not netted. The major classes of assets and liabilities classified as held for sale are separately disclosed in the notes to financial statements. (See Note 3.)