Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Litigation Settlement, Amount Awarded from Other Party		$ 995,000
Successor [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	$ 1,834,387	1,907,035
Impact Of Sponsor Actions In Residential Mortgage Backed Securities Subrogation			$ 0
Successor [Member] | Random Samples [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Gross loss reserve before subrogation recoveries	1,366,483	1,351,640
Subrogation recoveries	1,834,387	1,907,035
Gross loss reserve after subrogation recoveries	(467,904)	(555,395)
Random Samples [Member] | Successor [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	1,834,387	1,907,035	2,829,575	$ 2,523,540
Impact Of Sponsor Actions In Residential Mortgage Backed Securities Subrogation	0	(995,000)
Other Changes Rmbs Subrogation	$ (72,648)	$ 72,460	$ 306,035