Insurance Contracts - Additional Information (Details) $ in Thousands, € in Millions, £ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2022 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2021	Dec. 31, 2021 USD ($)	Mar. 31, 2022 GBP (£)	Mar. 31, 2022 EUR (€)	Mar. 31, 2021 GBP (£)	Mar. 31, 2021 EUR (€)	Dec. 31, 2020 USD ($)
Insurance [Line Items]
Reinsurance Recoverable Credit Exposure	$ 30,000			$ 31,000
Reinsurance Recoverable, Allowance for Credit Loss	49			10
Public Finance Puerto Rico Net Par Outstanding	784,000
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	1,502,000			1,730,000
Financial Guarantee Insurance Contracts, Premium Receivable	$ 308,000	$ 356,000		$ 320,000					$ 370,000
Estimated Future Premium Payments Weighted Average Discounted Rate	2.60%			2.20%	2.60%	2.60%
Reinsurance Payable	$ 40,000			$ 33,000
Weighted average period of future premiums	8 years 2 months 12 days			8 years
Uncollectable premium receivables	$ 8,000			$ 9,000
Accelerated premium revenue for retired obligations	0	0
Reinsurance recoveries of losses included in losses and loss expenses	(7,000)	(1,000)
Subrogation recoveries, net of reinsurance	(1,480,000)			(1,704,000)
Amortization of intangible assets	14,000	19,000
Intangible assets	350,000			362,000
Possible Increase in Loss Reserves Related to Puerto Rico	220,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	974,000	1,123,000		$ 1,098,000					$ 1,140,000
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves	754,000
Stockholders Equity After Possible Inability to Realize R&W Subrogation Recoveries	(505,000)
Uncollateralized Credit Exposure Supported by Unlimited Uncapped Indemnity	29,000
Past Due Premiums	1,000	0
Financial Guaranty
Insurance [Line Items]
Losses and loss expense reserves ceded to reinsurers	$ 14,000
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves	2.40%		1.20%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 101,000	122,000			£ 77		£ 89
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 16,000	$ 21,000				€ 15		€ 18