Financial Guarantee Insurance Contracts - Additional Information (Detail) € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 GBP (£)	Mar. 31, 2019 USD ($)	Feb. 12, 2019	Jun. 30, 2018 EUR (€)	Jun. 30, 2018 GBP (£)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Insurance [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	$ 1,762,217		$ 1,762,217		$ 1,770,546
Financial Guarantee Insurance Contracts, Premium Receivable	$ 441,526	$ 553,958	$ 441,526	$ 553,958	$ 495,391								$ 586,312
Estimated Future Premium Payments Weighted Average Discounted Rate	2.50%		2.50%		2.70%	2.50%	2.50%
Reinsurance Payable	$ 30,795		$ 30,795		$ 32,913
Weighted average period of future premiums			8 years 8 months 12 days		8 years 8 months 12 days
Uncollectable premium receivables	8,288		$ 8,288		$ 7,136
Past due premiums on policies insuring non-investment grade obligations amount	200		200
Accelerated premium revenue for retired obligations	(6,169)	6,103	6,054	15,495
Reinsurance recoveries of losses included in losses and loss expenses			(6,067)	870
Losses and loss expense reserves ceded to reinsurers	26,962	38,293	26,962	38,293	22,623								40,658
Subrogation recoveries, net of reinsurance	(1,736,055)		(1,736,055)		(1,744,243)
Intangible amortization expense	226,242	23,242	262,520	51,878
Intangible Assets, Gross (Excluding Goodwill)	1,265,052		1,265,052		1,551,576
Finite-Lived Intangible Assets, Accumulated Amortization	810,222		810,222		832,645
Insurance intangible asset	454,830		454,830		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	1,552,916	2,063,921	1,552,916	2,063,921	$ 1,633,147			$ 1,665,888				$ 2,109,165	$ 1,645,258
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves	452,916		452,916
Percent Cash & New COFINA Bonds Allocated Under POA									93.00%
Ambac Assurance Remaining Policy Obligations	177,605		177,605
Gain Contingency, Unrecorded Amount	142,000		$ 142,000
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves			2.10%		2.80%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	137,137	151,316	$ 137,137	151,316			£ 107,974				£ 114,729
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	28,367	$ 33,114	28,367	$ 33,114		€ 24,941				€ 28,380
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses			$ 119,560
External Credit Rating, Non Investment Grade [Member]
Insurance [Line Items]
Transactions with non-investment grade internal ratings			13.00%		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