Insurance Contracts - Additional Information (Details) $ in Thousands, € in Millions, £ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 GBP (£)	Jun. 30, 2024 EUR (€)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2023 GBP (£)	Jun. 30, 2023 EUR (€)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Insurance [Line Items]
Reinsurance Recoverable Credit Exposure	$ 176,000		$ 176,000					$ 128,000
Estimated Future Premium Payments Weighted Average Discounted Rate	3.20%		3.20%		3.20%	3.20%		3.20%
Reinsurance Payable	$ 140,000		$ 140,000					$ 90,000
Weighted average period of future premiums			7 years 7 months 6 days	8 years
Uncollectable premium receivables	3,000		$ 3,000					4,000
Reinsurance recoveries of losses included in losses and loss expenses			(98,000)	$ (45,000)
Amortization of Intangible Assets	8,000	$ 7,000	21,000	13,000
Intangible assets, less accumulated amortization	285,000		285,000					307,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,419,000	1,303,000	1,419,000	1,303,000			$ 1,418,000	$ 1,415,000			$ 1,307,000	$ 1,305,000
Past Due Premiums	1,000	0	$ 1,000	$ 0
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves			4.40%	4.60%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	70,000	76,000	$ 70,000	$ 76,000	£ 55				£ 60
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 12,000	$ 14,000	$ 12,000	$ 14,000		€ 11				€ 12