Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail) - USD ($) $ in Millions	Jun. 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	$ 1,220
Standard & Poor's, B- Rating [Member]
Schedule of Investments [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 1,228	$ 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