Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - Successor [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 1,907,035
Changes recognized
Impact of sponsor actions			$ 0
Discounted RMBS subrogation (gross of reinsurance), ending balance	1,834,387	$ 1,907,035
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,907,035	2,829,575	2,523,540
Changes recognized
Impact of sponsor actions	0	(995,000)
Other Changes Rmbs Subrogation	(72,648)	72,460	306,035
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,834,387	$ 1,907,035	$ 2,829,575