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) - Successor [Member] - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 66,940	$ 50,105	$ 52,070	$ 31,176
Additions for credit impairments recognized on:
Securities not previously impaired	3,274	2,257
Securities previously impaired	11,596	16,672
Reductions for credit impairments previously recognized on:
Balance at end	$ 66,940	$ 50,105	$ 52,070	$ 31,176