Condensed Consolidated Statements of Cash Flows (unaudited) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Statement of Cash Flows [Abstract]
Net cash flows provided by (used in) operating activities	$ (466)	$ (164)
Fixed-maturity securities:
Purchases	(396)	(278)
Sales	120	86
Maturities and paydowns	225	217
Short-term investments with maturities of over three months:
Purchases	0	(56)
Sales	0	4
Maturities and paydowns	30	12
Net sales (purchases) of short-term investments with original maturities of less than three months	120	375
Paydowns on financial guaranty variable interest entities’ assets	14	37
Purchases of other invested assets	(32)	(10)
Sales and return of capital of other invested assets	45	1
Other	(1)	1
Net cash flows provided by (used in) investing activities	125	389
Financing activities
Dividends paid	(18)	(20)
Repurchases of common shares	(77)	(116)
Net paydowns of financial guaranty variable interest entities’ liabilities	(13)	(35)
Paydown of long-term debt	0	(21)
Other	7	(11)
Proceeds from issuance of collateralized loan obligations	752	0
Proceeds from issuance of warehouse financing debt	217	0
Repayment of warehouse financing debt	(476)	0
Contributions from noncontrolling interests to investment vehicles	3	23
Distributions to noncontrolling interests from investment vehicles	(6)	0
Net cash flows provided by (used in) financing activities	389	(180)
Effect of foreign exchange rate changes	0	(7)
Increase (decrease) in cash and restricted cash	48	38
Cash and restricted cash at beginning of period	298	183	$ 183
Supplemental cash flow information
Interest on long-term debt	8	9
Cash	95	139	162
Restricted cash (included in other assets)	1	0
Cash of consolidated investment vehicles (see Note 9)	250	82
Cash and restricted cash at end of period	$ 346	$ 221	$ 298