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		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
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	$ 560	$ 442	$ 522	$ 411	$ 411
Accretion of discount	6	3	15	8
Changes in discount rates	(12)	(25)	(8)	(114)
Changes in timing and assumptions	93	(50)	140	(42)
Total economic loss development (benefit)	87	(72)	147	(148)
Net (paid) recovered losses	(157)	357	(179)	464
Net expected loss to be paid (recovered), end of period	490	727	$ 490	727	$ 522
Weighted average risk free discount rate			4.80%		4.08%
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)			45 days
Minimum
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Risk free discount rate			4.51%		3.82%
Maximum
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Risk free discount rate			5.45%		4.69%
United States
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid after recoveries for representations and warranties, percent			98.20%		98.50%
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	443	217	$ 412	209	$ 209
Total economic loss development (benefit)	134	22	193	(22)
Net (paid) recovered losses	(169)	393	(197)	445
Net expected loss to be paid (recovered), end of period	408	632	408	632	412
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	433	210	403	197	197
Total economic loss development (benefit)	135	24	193	(16)
Net (paid) recovered losses	(169)	392	(197)	445
Net expected loss to be paid (recovered), end of period	399	626	399	626	403
Public finance | 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	10	7	9	12	12
Total economic loss development (benefit)	(1)	(2)	0	(6)
Net (paid) recovered losses	0	1	0	0
Net expected loss to be paid (recovered), end of period	9	6	9	6	9
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	73	179	66	150	150
Total economic loss development (benefit)	(48)	(95)	(52)	(127)
Net (paid) recovered losses	13	(32)	24	29
Net expected loss to be paid (recovered), end of period	38	52	38	52	66
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	44	46	44	52	52
Total economic loss development (benefit)	1	1	6	1
Net (paid) recovered losses	(1)	(4)	(6)	(10)
Net expected loss to be paid (recovered), end of period	44	43	44	43	44
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	117	225	110	202	202
Total economic loss development (benefit)	(47)	(94)	(46)	(126)
Net (paid) recovered losses	12	(36)	18	19
Net expected loss to be paid (recovered), end of period	$ 82	$ 95	$ 82	$ 95	$ 110