Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 3	$ 0