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) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 12
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	1
Balance at end	$ 12