Financial Guarantee Insurance Contracts - Additional Information (Details) $ in Thousands, € in Millions, £ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Mar. 31, 2020	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2020 GBP (£)	Sep. 30, 2020 EUR (€)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 GBP (£)	Sep. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Insurance [Line Items]
Reinsurance Recoverable Credit Exposure	$ 1,000			$ 1,000
Reinsurance Recoverable, Allowance for Credit Loss	0			0
Public Finance Puerto Rico Net Par Outstanding	1,070,000			1,070,000
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	1,757,000			1,757,000		$ 1,727,000
Financial Guarantee Insurance Contracts, Premium Receivable	$ 372,000		$ 415,000	$ 372,000	$ 415,000	$ 416,000							$ 495,000
Estimated Future Premium Payments Weighted Average Discounted Rate	2.20%			2.20%		2.40%	2.20%	2.20%
Reinsurance Payable	$ 27,000			$ 27,000		$ 29,000
Weighted average period of future premiums				8 years 6 months		8 years 6 months
Uncollectable premium receivables	18,000			$ 18,000
Accelerated premium revenue for retired obligations	5,000		2,000	6,000	8,000
Reinsurance recoveries of losses included in losses and loss expenses				(14,000)	(6,000)
Losses and loss expense reserves ceded to reinsurers	36,000		26,000	36,000	26,000	$ 26,000							23,000
Subrogation recoveries, net of reinsurance	(1,731,000)			(1,731,000)		(1,702,000)
Amortization of insurance intangible assets	14,000		17,000	41,000	280,000
Intangible Assets, Gross (Excluding Goodwill)	1,268,000			1,268,000		1,273,000
Finite-Lived Intangible Assets, Accumulated Amortization	885,000			885,000		847,000
Insurance intangible asset	383,000			383,000		427,000
Possible Increase in Loss Reserves Related to Puerto Rico	1,200,000			1,200,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,095,000		1,629,000	1,095,000	1,629,000	$ 1,536,000			$ 1,129,000			$ 1,553,000	$ 1,633,000
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves	(105,000)			(105,000)
Stockholders Equity After Possible Inability to Realize R&W Subrogation Recoveries	(636,000)			$ (636,000)
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves		2.10%		0.90%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	112,000		124,000	$ 112,000	124,000		£ 87			£ 101
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	21,000		$ 22,000	21,000	$ 22,000			€ 18			€ 20
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses	43,000			263,000
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