Expected Loss to be Paid (Recovered) - Net Expected Loss to be Paid (Recovered) Roll Forward by Sector (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid (recovered), beginning of period	$ 529	$ 737	$ 1,183
Economic Loss Development (Benefit)	(287)	145	(1)
Net (paid) recovered losses	169	(353)	(445)
Net expected loss to be paid (recovered), end of period	$ 411	529	737
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)	45 days
Loss and LAE reserve paid	$ 36	25	35
Expected LAE to be paid	26	23
Ceded expected loss to be recovered	10	23
Public finance
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid (recovered), beginning of period	341	554	864
Economic Loss Development (Benefit)	(204)	203	215
Net (paid) recovered losses	72	(416)	(525)
Net expected loss to be paid (recovered), end of period	209	341	554
Other structured finance
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid (recovered), beginning of period	40	37	26
Economic Loss Development (Benefit)	17	13	18
Net (paid) recovered losses	(5)	(10)	(7)
Net expected loss to be paid (recovered), end of period	52	40	37
Structured finance
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid (recovered), beginning of period	188	183	319
Economic Loss Development (Benefit)	(83)	(58)	(216)
Net (paid) recovered losses	97	63	80
Net expected loss to be paid (recovered), end of period	202	188	183
U.S. | Public finance
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid (recovered), beginning of period	305	531	832
Economic Loss Development (Benefit)	(182)	190	224
Net (paid) recovered losses	74	(416)	(525)
Net expected loss to be paid (recovered), end of period	197	305	531
U.S. | RMBS
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid (recovered), beginning of period	148	146	293
Economic Loss Development (Benefit)	(100)	(71)	(234)
Net (paid) recovered losses	102	73	87
Net expected loss to be paid (recovered), end of period	150	148	146
Non U.S. | Public finance
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid (recovered), beginning of period	36	23	32
Economic Loss Development (Benefit)	(22)	13	(9)
Net (paid) recovered losses	(2)	0	0
Net expected loss to be paid (recovered), end of period	$ 12	$ 36	$ 23