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, 2020	Dec. 31, 2019
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	$ 1,253
Standard & Poor's, B- Rating [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,260	$ 1,164
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		1,153
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	6	11
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, 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	310
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	318	186
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		176
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	6	11
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
Corporate Debt 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	470
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	470	535
Corporate Debt 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		535
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
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	473
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	$ 473	442
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		$ 442