Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 4	$ 6
Standard & Poor's, C Rating | Assured Guaranty Municipal Corporation
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1	1
Standard & Poor's, CCC+ Rating
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,257	1,273
Standard & Poor's, CCC+ Rating | Ambac Assurance Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,252	1,266
Municipal Bonds [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	4	6
Municipal Bonds [Member] | Standard & Poor's, C Rating | Assured Guaranty Municipal Corporation
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1	1
Municipal Bonds [Member] | Standard & Poor's, CCC+ Rating
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	328	327
Municipal Bonds [Member] | Standard & Poor's, CCC+ Rating | Ambac Assurance Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	323	320
Corporate Debt Securities [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0	0
Corporate Debt Securities [Member] | Standard & Poor's, C Rating | Assured Guaranty Municipal Corporation
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0	0
Corporate Debt Securities [Member] | Standard & Poor's, CCC+ Rating
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	464	465
Corporate Debt Securities [Member] | Standard & Poor's, CCC+ Rating | Ambac Assurance Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	464	465
Mortgage And Other Asset Backed Securities [Member] | Standard & Poor's, CCC+ Rating
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	465	481
Mortgage And Other Asset Backed Securities [Member] | Standard & Poor's, CCC+ Rating | Ambac Assurance Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 465	$ 481