Insurance Contracts - Additional Information (Details) $ in Thousands, € in Millions, £ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2024 GBP (£)	Sep. 30, 2024 EUR (€)	Jun. 30, 2024 USD ($)	Sep. 30, 2023 GBP (£)	Sep. 30, 2023 EUR (€)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Insurance [Line Items]
Reinsurance Recoverable Credit Exposure	$ 200,000		$ 200,000		$ 128,000
Estimated Future Premium Payments Weighted Average Discounted Rate	3.10%		3.10%		3.20%	3.10%	3.10%
Reinsurance Payable	$ 165,000		$ 165,000		$ 90,000
Weighted average period of future premiums			7 years 4 months 24 days		8 years
Uncollectable premium receivables	3,000		$ 3,000		$ 4,000
Reinsurance recoveries of losses included in losses and loss expenses			(74,000)	$ (75,000)
Amortization of Intangible Assets	13,000	$ 7,000	33,000	21,000
Intangible assets, less accumulated amortization	598,000		598,000		307,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,670,000	1,318,000	1,670,000	1,318,000	$ 1,415,000			$ 1,419,000			$ 1,303,000	$ 1,305,000
Past Due Premiums	3,000	2,000	$ 3,000	$ 2,000
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves			3.80%	4.60%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	67,000	71,000	$ 67,000	$ 71,000		£ 50			£ 58
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 12,000	$ 13,000	$ 12,000	$ 13,000			€ 11			€ 13