Insurance Contracts - Additional Information (Details) $ in Thousands, € in Millions, £ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Jun. 30, 2022	Dec. 31, 2022 USD ($)	Mar. 31, 2023 GBP (£)	Mar. 31, 2023 EUR (€)	Mar. 31, 2022 GBP (£)	Mar. 31, 2022 EUR (€)	Dec. 31, 2021 USD ($)
Insurance [Line Items]
Reinsurance Recoverable Credit Exposure	$ 75,000			$ 60,000
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0			140,000
Financial Guarantee Insurance Contracts, Premium Receivable	$ 256,000	$ 308,000		$ 254,000					$ 320,000
Estimated Future Premium Payments Weighted Average Discounted Rate	3.00%			3.00%	3.00%	3.00%
Reinsurance Payable	$ 38,000			$ 39,000
Weighted average period of future premiums	8 years			8 years
Uncollectable premium receivables	$ 5,000			$ 5,000
Accelerated premium revenue for retired obligations	0	0
Reinsurance recoveries of losses included in losses and loss expenses	(20,000)	(7,000)
Subrogation recoveries, net of reinsurance	0			(140,000)
Amortization of Intangible Assets	7,000	14,000
Intangible assets	321,000			326,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,307,000	974,000		$ 1,305,000					$ 1,098,000
Past Due Premiums	$ 0	1,000
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves	3.50%		3.90%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 75,000	101,000			£ 61		£ 77
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 15,000	$ 16,000				€ 13		€ 15