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	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 12	$ 12
Additions for credit impairments recognized on:
Securities not previously impaired	0
Securities previously impaired	0
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	0
Balance at end	$ 12	$ 12