Expected Loss to be Paid (Recovered) - Net Expected Loss to be Paid After Net Expected Recoveries for Breaches of R&W Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
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	$ 466	$ 735	$ 529	$ 737
Accretion of discount	2	1	5	7
Changes in discount rates	(1)	(2)	(34)	30
Changes in timing and assumptions	(95)	71	(72)	64
Total economic loss development (benefit)	(94)	70	(101)	101
Net (paid) recovered losses	(173)	(334)	(229)	(367)
Net expected loss to be paid (recovered), end of period	199	471	$ 199	471
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	5	11	$ 15	20
Expected LAE to be paid	27		27		$ 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	243	572	341	554
Total economic loss development (benefit)	(31)	60	(28)	151
Net (paid) recovered losses	(202)	(336)	(303)	(409)
Net expected loss to be paid (recovered), end of period	10	296	10	296
Public finance | United States
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	221	543	305	531
Total economic loss development (benefit)	(29)	56	(13)	142
Net (paid) recovered losses	(201)	(336)	(301)	(410)
Net expected loss to be paid (recovered), end of period	(9)	263	(9)	263
Public finance | Non United States
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	22	29	36	23
Total economic loss development (benefit)	(2)	4	(15)	9
Net (paid) recovered losses	(1)	0	(2)	1
Net expected loss to be paid (recovered), end of period	19	33	19	33
RMBS | United States
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	178	128	148	146
Total economic loss development (benefit)	(65)	1	(82)	(61)
Net (paid) recovered losses	29	8	76	52
Net expected loss to be paid (recovered), end of period	142	137	142	137
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	45	35	40	37
Total economic loss development (benefit)	2	9	9	11
Net (paid) recovered losses	0	(6)	(2)	(10)
Net expected loss to be paid (recovered), end of period	47	38	47	38
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	223	163	188	183
Total economic loss development (benefit)	(63)	10	(73)	(50)
Net (paid) recovered losses	29	2	74	42
Net expected loss to be paid (recovered), end of period	$ 189	$ 175	$ 189	$ 175