Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	$ 0	$ 9
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	2	6
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] | Municipal Bonds [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	2	6
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] | Corporate Debt Securities [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 0	$ 0