Insurance Contracts - Additional Information (Details) € in Millions, £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Mar. 31, 2024 GBP (£)	Mar. 31, 2024 EUR (€)	Dec. 31, 2023 USD ($)	Mar. 31, 2023 GBP (£)	Mar. 31, 2023 EUR (€)	Dec. 31, 2022 USD ($)
Insurance [Line Items]
Reinsurance Recoverable Credit Exposure	$ 156				$ 128
Estimated Future Premium Payments Weighted Average Discounted Rate	3.20%		3.20%	3.20%	3.20%
Reinsurance Payable	$ 110				$ 90
Weighted average period of future premiums	7 years 8 months 12 days	8 years
Uncollectable premium receivables	$ 3				4
Reinsurance recoveries of losses included in losses and loss expenses	(35)	$ (20)
Amortization of Intangible Assets	12	7
Intangible assets, less accumulated amortization	293				307
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,418	1,307			$ 1,415			$ 1,305
Past Due Premiums	$ 503	$ 0
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves	4.20%	4.60%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 71	$ 75	£ 56			£ 61
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 13	$ 15		€ 12			€ 13