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 Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 11,680	$ 14,186	$ 12,454	$ 67,085
Additions for credit impairments recognized on:
Securities not previously impaired	0	226
Securities previously impaired	0	97
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	774	53,222
Balance at end	$ 11,680	$ 14,186	$ 12,454	$ 67,085