Variable Interest Entities - Additional Information (Details) € in Millions			3 Months Ended			12 Months Ended
	Mar. 31, 2021 USD ($) policy Entity	Dec. 23, 2020 USD ($)	Mar. 31, 2021 USD ($) fund policy Entity	Mar. 31, 2021 USD ($) policy Entity	Mar. 31, 2020 USD ($) Entity	Dec. 31, 2020 USD ($) Entity policy	Dec. 31, 2020 EUR (€)
Variable Interest Entity [Line Items]
Net change in fair value of credit derivatives			$ (19,000,000)		$ (77,000,000)
Proceeds from issuance of warehouse financing debt			217,000,000		$ 0
Other consolidated VIE assets	$ 95,000,000		95,000,000	$ 95,000,000		$ 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	$ 16,018,000,000		$ 16,018,000,000	$ 16,018,000,000		$ 15,334,000,000
Liabilities	9,525,000,000		9,525,000,000	9,525,000,000		8,629,000,000
Variable Interest Entity, Primary Beneficiary | Line of Credit | CLO XXXI
Variable Interest Entity [Line Items]
Commitment percentage		75.00%
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Assets	178,000,000		178,000,000	178,000,000		204,000,000
Liabilities	51,000,000		51,000,000	51,000,000		$ 9,000,000
Net fair value gains and losses on FG VIEs are expected to reverse to zero at maturity of the VIE debt	71,000,000		71,000,000	71,000,000
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	25		25	25		25
FG VIE matured in the period, number | Entity					1
Assets	$ 281,000,000		$ 281,000,000	$ 281,000,000		$ 296,000,000
Liabilities	318,000,000		318,000,000	318,000,000		333,000,000
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			$ 3,000,000		$ 3,000,000
Consolidated Investment Vehicles | AssuredIM Fund
Variable Interest Entity [Line Items]
Number of consolidated investment managed funds | fund			6
Number Of Financial Assets Deconsolidated | fund			1
Consolidated Investment Vehicles | CLOs and CLO warehouse assets
Variable Interest Entity [Line Items]
Number of consolidated investment managed collateralized loan obligations | fund			5
Number Of Financial Assets Consolidated | fund			1
Consolidated Investment Vehicles | CLO Warehouse
Variable Interest Entity [Line Items]
Number of consolidated investment managed collateralized loan obligations | fund			1
Number Of Financial Assets Consolidated | fund			1
Number Of Financial Assets Securitized | fund			1
Consolidated Investment Vehicles | Foreign Exchange Forward
Variable Interest Entity [Line Items]
Collateral posting requirement	54,000,000		$ 54,000,000	54,000,000		11,000,000
Consolidated Investment Vehicles | Interest Rate Swap
Variable Interest Entity [Line Items]
Collateral posting requirement	13,000,000		13,000,000	13,000,000		8,000,000
Consolidated Investment Vehicles | Forward Contracts And Interest Rate Swaps
Variable Interest Entity [Line Items]
Net change in fair value of credit derivatives			2,000,000
Consolidated Investment Vehicles | Maximum | Future Equity Investments
Variable Interest Entity [Line Items]
Committed capital			7,000,000			6,000,000
Consolidated Investment Vehicles | Average | Foreign Exchange Forward
Variable Interest Entity [Line Items]
Collateral posting requirement	33,000,000		33,000,000	33,000,000		6,000,000
Consolidated Investment Vehicles | Average | Interest Rate Swap
Variable Interest Entity [Line Items]
Collateral posting requirement	10,000,000		$ 10,000,000	10,000,000		4,000,000
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Line of Credit | CLO XXXI
Variable Interest Entity [Line Items]
Maximum borrowing capacity		$ 360,000,000
Commitment amount	$ 90,000,000
Proceeds from issuance of warehouse financing debt				$ 68,000,000
Consolidated Investment Vehicles | Variable Interest Entity, Primary Beneficiary | Line of Credit | CLO XXXI | London Interbank Offered Rate (LIBOR)
Variable Interest Entity [Line Items]
Basis spread on variable rate		130.00%
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