Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Feb. 12, 2018
Loss And Loss Adjustment Expense Reserves [Line Items]
Impact of VIE Consolidation on Gross Loss and Loss Reserves			$ (72,159)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	$ (106,882)	$ 4,113,802
Less reinsurance on loss and loss expense reserves	22,623	40,658
Beginning balance of net loss and loss expense reserves	(129,505)	4,073,144
Current Year Claims and Claims Adjustment Expense	659	976
Prior Year Claims and Claims Adjustment Expense	(84,593)	(182,291)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	(83,934)	(181,315)
Claim and loss expense payments, net of subrogation and reinsurance	(32)	(143)
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(298,909)	(3,937,561)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	298,941	3,937,704
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	(1,174)	(9,578)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(129,505)	4,073,144	(513,554)	$ (129,505)	$ (55,453)
Add reinsurance on loss and loss expense reserves	25,749	25,326
Ending gross loss and loss expense reserves	(559,964)	(30,127)
Policyholder Benefits and Claims Incurred, Ceded	5,999	123
Net Incurred RMBS Subrogation Recoveries	$ (15,365)	$ (56,928)
satisfaction and discharge of segregated account deferred interest amounts						$ 856,834
Loss and Loss Expense Benefit on Settled Deferred Amounts						$ 288,204
Ceded Loss And Loss Expenses Paid Not Yet Recovered			$ 287	$ 510	$ 185