Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance	$ (1,816,289)		$ (1,878,740)
Successor [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	1,844,116		1,907,035
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	1,310,563		1,351,640
Subrogation recoveries	1,844,116		1,907,035
Gross loss reserve after subrogation recoveries	(533,553)		(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	1,844,116	$ 1,923,256	$ 1,907,035	$ 2,829,575
Impact Of Sponsor Actions In Residential Mortgage Backed Securities Subrogation	0	(995,000)
Other Changes Rmbs Subrogation	$ (62,919)	$ 88,681