Variable Interest Entities - Additional Information (Details) € in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) policy Entity	Sep. 30, 2021 USD ($) policy Entity	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) fund Entity policy	Sep. 30, 2020 USD ($) Entity	Dec. 31, 2020 USD ($) Entity policy	Dec. 31, 2020 EUR (€)
Variable Interest Entity [Line Items]
Fair value gains (losses) on credit derivatives		$ 21,000,000	$ (3,000,000)	$ (31,000,000)	$ 20,000,000
Proceeds from issuance of warehouse financing debt				1,008,000,000	$ 47,000,000
Other consolidated VIE assets	$ 96,000,000	96,000,000		96,000,000		$ 96,000,000
Other consolidated VIE liabilities	$ 9,000,000	$ 9,000,000		$ 9,000,000		$ 3,000,000
Number of policies monitored | policy	17,000	17,000		17,000
Number of policies monitored, not within the scope of ASC 810 | policy	15,000	15,000		15,000
Number of policies that contain provisions for consolidation | policy	80	80		80		79
Assets	$ 17,614,000,000	$ 17,614,000,000		$ 17,614,000,000		$ 15,334,000,000
Liabilities	11,218,000,000	11,218,000,000		11,218,000,000		8,629,000,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	214,000,000	214,000,000		214,000,000		204,000,000
Liabilities	60,000,000	60,000,000		60,000,000		$ 9,000,000
Net fair value gains and losses on FG VIEs are expected to reverse to zero at maturity of the VIE debt	102,000,000	102,000,000		102,000,000
Financial Guaranty Variable Interest Entities
Variable Interest Entity [Line Items]
Maximum loss exposure	$ 0	$ 0		$ 0
Financial Guaranty Variable Interest Entities | Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Total number of entities consolidated | Entity	24	24		24		25
FG VIEs deconsolidated in the period | Entity				1
FG VIE matured in the period, number | Entity					2
FG VIEs consolidated in the period | Entity					2
Loss on consolidation					$ 1,000,000
Assets	$ 271,000,000	$ 271,000,000		$ 271,000,000		$ 296,000,000
Financial Guaranty Variable Interest Entities | Variable Interest Entity, Primary Beneficiary | Residential Mortgage Backed Securities and Other Insurance Products
Variable Interest Entity [Line Items]
Change in the instrument specific credit risk of the VIEs' assets		5,000,000	$ 14,000,000	$ 9,000,000	$ (2,000,000)
Consolidated Investment Vehicles
Variable Interest Entity [Line Items]
Number of financial assets consolidated | fund				6
Number of financial assets deconsolidated | fund				1
Consolidated Investment Vehicles | Foreign Exchange Forward
Variable Interest Entity [Line Items]
Collateral posting requirement	26,000,000	26,000,000		$ 26,000,000		11,000,000
Consolidated Investment Vehicles | Interest Rate Swap
Variable Interest Entity [Line Items]
Collateral posting requirement	23,000,000	23,000,000		23,000,000		8,000,000
Consolidated Investment Vehicles | Forward Contracts And Interest Rate Swaps
Variable Interest Entity [Line Items]
Fair value gains (losses) on credit derivatives				1,000,000
Consolidated Investment Vehicles | Maximum | Future Equity Investments
Variable Interest Entity [Line Items]
Committed capital				17,000,000		6,000,000
Consolidated Investment Vehicles | Average | Foreign Exchange Forward
Variable Interest Entity [Line Items]
Collateral posting requirement	19,000,000	19,000,000		19,000,000		6,000,000
Consolidated Investment Vehicles | Average | Interest Rate Swap
Variable Interest Entity [Line Items]
Collateral posting requirement	15,000,000	15,000,000		$ 15,000,000		4,000,000
Consolidated Investment Vehicles | CLO Warehouse
Variable Interest Entity [Line Items]
Number of financial assets securitized | fund				3
Number of consolidated investment managed collateralized loan obligations | fund				2
Consolidated Investment Vehicles | AssuredIM Fund
Variable Interest Entity [Line Items]
Number of consolidated investment managed funds | fund				7
Consolidated Investment Vehicles | CLOs
Variable Interest Entity [Line Items]
Number of consolidated investment managed collateralized loan obligations | fund				7
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Line of Credit
Variable Interest Entity [Line Items]
Maximum borrowing capacity	777,000,000	777,000,000		$ 777,000,000
Remaining borrowing capacity	279,000,000	$ 279,000,000		$ 279,000,000
Proceeds from issuance of warehouse financing debt	$ 216,000,000
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Line of Credit | London Interbank Offered Rate (LIBOR)
Variable Interest Entity [Line Items]
Basis spread on variable rate	120.00%
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Line of Credit | Euribor
Variable Interest Entity [Line Items]
Basis spread on variable rate	85.00%
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Line of Credit | EUR 2021-1
Variable Interest Entity [Line Items]
Proceeds from issuance of warehouse financing debt						25,000,000	€ 20
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Line of Credit | Assured IM
Variable Interest Entity [Line Items]
Proceeds from issuance of warehouse financing debt						$ 1,000,000	€ 1