Financial Guarantee Insurance Contracts - Additional Information (Detail) € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 EUR (€)	Sep. 30, 2019 GBP (£)	Jun. 30, 2019 USD ($)	Feb. 12, 2019	Sep. 30, 2018 EUR (€)	Sep. 30, 2018 GBP (£)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Insurance [Line Items]
Public Finance Puerto Rico Net Par Outstanding	$ 1,123,000		$ 1,123,000
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	1,754,944		1,754,944		$ 1,770,546
Financial Guarantee Insurance Contracts, Premium Receivable	$ 414,744	$ 517,197	$ 414,744	$ 517,197	$ 495,391								$ 586,312
Estimated Future Premium Payments Weighted Average Discounted Rate	2.40%		2.40%		2.70%	2.40%	2.40%
Reinsurance Payable	$ 29,950		$ 29,950		$ 32,913
Weighted average period of future premiums			8 years 7 months 6 days		8 years 8 months 12 days
Uncollectable premium receivables	13,943		$ 13,943		$ 7,136
Past due premiums on policies insuring non-investment grade obligations amount	100		100
Accelerated premium revenue for retired obligations	2,019	6,751	8,073	22,246
Reinsurance recoveries of losses included in losses and loss expenses			(5,999)	(123)
Losses and loss expense reserves ceded to reinsurers	25,749	25,326	25,749	25,326	22,623								40,658
Subrogation recoveries, net of reinsurance	(1,728,879)		(1,728,879)		(1,744,243)
Intangible amortization expense	17,421	26,421	279,941	78,299
Intangible Assets, Gross (Excluding Goodwill)	1,258,362		1,258,362		1,551,576
Finite-Lived Intangible Assets, Accumulated Amortization	824,838		824,838		832,645
Insurance intangible asset	433,524		433,524		718,931
Possible Increase in Loss Reserves Related to Puerto Rico	1,100,000		1,100,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,628,664	1,798,809	1,628,664	1,798,809	$ 1,633,147			$ 1,552,916				$ 2,063,921	$ 1,645,258
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves	528,664		528,664
Percent Cash & New COFINA Bonds Allocated Under POA									93.00%
Ambac Assurance Remaining Policy Obligations	101,584		$ 101,584
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves			1.80%		2.80%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	124,406	136,925	$ 124,406	136,925			£ 101,168				£ 104,988
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	22,091	$ 32,613	22,091	$ 32,613		€ 20,264				€ 28,085
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses			$ 196,599
External Credit Rating, Non Investment Grade [Member]
Insurance [Line Items]
Transactions with non-investment grade internal ratings			14.00%		20.00%
Long-term Debt [Member]
Insurance [Line Items]
Gain Contingency, Unrecorded Amount	1,600,000		$ 1,600,000
Long-term Debt [Member]
Insurance [Line Items]
Gain Contingency, Unrecorded Amount	$ 1,400,000		$ 1,400,000