Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) € in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2016 USD ($)	Dec. 31, 2016 EUR (€)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less reinsurance on loss and loss expense reserves	$ 30,767
Prior years:
Add reinsurance on loss and loss expense reserves	40,658		$ 30,767
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance						$ (1,806,736)	$ (1,878,740)
Successor [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	28,939		(77,578)		$ (24,831)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	513,186		(11,489)		(768,707)
Ceded Loss And Loss Expenses Paid Not Yet Recovered						339	(349)	$ (60)
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation						(1,834,387)	(1,907,035)
Policyholder Benefits and Claims Incurred, Ceded	20,348		(5,421)		(47,085)
Net Incurred RMBS Subrogation Recoveries	(72,003)		71,369		303,633
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	3,696,038		2,858,813		3,798,733
Less reinsurance on loss and loss expense reserves	30,767		44,059		100,355
Beginning balance of net loss and loss expense reserves	3,665,271		2,814,754		3,698,378
Current year:
Claim and loss expense payments, net of subrogation and reinsurance	(825)		(5,371)		0
Prior years:
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(133,427)		944,955		(90,086)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,665,271		2,814,754		3,698,378	$ 4,073,144	$ 3,665,271	$ 2,814,754
Add reinsurance on loss and loss expense reserves	40,658		30,767		44,059
Ending gross loss and loss expense reserves	4,113,802		3,696,038		2,858,813
Current Year Claims and Claims Adjustment Expense	5,691		6,675		1,183
Prior Year Claims and Claims Adjustment Expense	507,495		(18,164)		(769,890)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	134,252		(939,584)		90,086
Euro Member Countries, Euro
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	21,375	€ 20,297	19,019	€ 17,515
Prior years:
Ending gross loss and loss expense reserves	$ 21,116	€ 17,582	$ 21,375	€ 20,297	$ 19,019