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	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 38,732	$ 54,362	$ 67,085	$ 52,070
Additions for credit impairments recognized on:
Securities not previously impaired	226	307
Securities previously impaired	98	1,985
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	28,677	0
Balance at end	$ 38,732	$ 54,362	$ 67,085	$ 52,070