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		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
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	$ 505	$ 522	$ 522
Accretion of discount	5	5
Changes in discount rates	(3)	10
Changes in timing and assumptions	(9)	(4)
Total economic loss development (benefit)	(7)	11
Net (paid) recovered losses	(65)	(16)
Net expected loss to be paid (recovered), end of period	$ 433	517	$ 505
Weighted average risk free discount rate	4.44%		4.10%
Period after the end of the reporting period within which the ceded paid losses are typically settled (in days)	45 days
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	$ 418	412	$ 412
Total economic loss development (benefit)	(3)	5
Net (paid) recovered losses	(17)	(24)
Net expected loss to be paid (recovered), end of period	398	393	418
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	44	44
Total economic loss development (benefit)	(1)	1
Net (paid) recovered losses	(6)	(3)
Net expected loss to be paid (recovered), end of period	37	42	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	87	110	110
Total economic loss development (benefit)	(4)	6
Net (paid) recovered losses	(48)	8
Net expected loss to be paid (recovered), end of period	$ 35	124	$ 87
Minimum
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Risk free discount rate	4.14%		3.79%
Maximum
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Risk free discount rate	5.33%		5.40%
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	95.50%		96.10%
United States | 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	$ 398	403	$ 403
Total economic loss development (benefit)	(3)	1
Net (paid) recovered losses	(17)	(24)
Net expected loss to be paid (recovered), end of period	378	380	398
United States | 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	43	66	66
Total economic loss development (benefit)	(3)	5
Net (paid) recovered losses	(42)	11
Net expected loss to be paid (recovered), end of period	(2)	82	43
Non-U.S. public finance | 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	20	9	9
Total economic loss development (benefit)	0	4
Net (paid) recovered losses	0	0
Net expected loss to be paid (recovered), end of period	$ 20	$ 13	$ 20