Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Standard & Poor's, B Rating [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 785
Standard & Poor's, B Rating [Member] | Ambac Assurance Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	783
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	2	$ 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
Standard & Poor's, CCC+ Rating
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		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		1,266
Standard & Poor's, A- Rating | Assured Guaranty Municipal Corporation
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1
Municipal Bonds [Member] | Standard & Poor's, B Rating [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	320
Municipal Bonds [Member] | Standard & Poor's, B Rating [Member] | Ambac Assurance Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	317
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	2	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
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		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		320
Municipal Bonds [Member] | Standard & Poor's, A- Rating | Assured Guaranty Municipal Corporation
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1
Corporate Debt Securities [Member] | Standard & Poor's, B Rating [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0
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
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		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		465
Corporate Debt Securities [Member] | Standard & Poor's, A- Rating | Assured Guaranty Municipal Corporation
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0
Mortgage And Other Asset Backed Securities [Member] | Standard & Poor's, B Rating [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	466
Mortgage And Other Asset Backed Securities [Member] | Standard & Poor's, B Rating [Member] | Ambac Assurance Corporation [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 466
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		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		$ 481