Basis of Presentation and Significant Accounting Policies - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	May 23, 2019	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			$ 0		$ 2
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings | Accounting Standards Update 2017-04 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				$ 0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				$ (18)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Retained Earnings | Section 4013 of the CARES Act [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 0
The Navigators Group, Inc. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Business Acquisition, Share Price	$ 70
Business Combination, Consideration Transferred	$ 2,137