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	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 12	$ 67
Additions for credit impairments recognized on:
Securities not previously impaired	0	1
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	(1)	(56)
Balance at end	$ 12	$ 12
Debt Securities, Available-for-sale, Allowance for Credit Loss			$ 0