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