Insurance Contracts - Additional Information (Details) € in Millions, £ in Millions, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2023 GBP (£)	Sep. 30, 2023 EUR (€)	Jun. 30, 2023 USD ($)	Sep. 30, 2022 GBP (£)	Sep. 30, 2022 EUR (€)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Insurance [Line Items]
Reinsurance Recoverable Credit Exposure	$ 110		$ 110		$ 60
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0		0		140
Financial Guarantee Insurance Contracts, Premium Receivable	$ 245	$ 259	$ 245	$ 259	$ 254							$ 320
Estimated Future Premium Payments Weighted Average Discounted Rate	3.20%		3.20%		3.00%	3.20%	3.20%
Reinsurance Payable	$ 95		$ 95		$ 39
Weighted average period of future premiums			7 years 9 months 18 days		8 years
Uncollectable premium receivables	4		$ 4		$ 5
Reinsurance recoveries of losses included in losses and loss expenses			(75)	(30)
Subrogation recoveries, net of reinsurance	0		0		(140)
Amortization of Intangible Assets	7	6	21	34
Intangible assets, less accumulated amortization	312		312		326
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,318	1,071	1,318	1,071	$ 1,305			$ 1,303			$ 846	$ 1,098
Past Due Premiums	2	0	$ 2	$ 0
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves			4.60%	3.90%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	71	73	$ 71	$ 73		£ 58			£ 66
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 13	$ 14	$ 13	$ 14			€ 13			€ 14