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, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
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	$ 186	$ 447	$ 106	$ 505	$ 505
Accretion of discount	2	5	5	14
Changes in discount rates	0	3	6	1
Changes in timing and assumptions	(40)	(42)	(28)	(35)
Total economic loss development (benefit)	(38)	(34)	(17)	(20)
Net (paid) recovered losses	(44)	(107)	15	(179)
Net expected loss to be paid (recovered), end of period	104	306	104	$ 306	106
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	192	411	116	$ 418	418
Total economic loss development (benefit)	(8)	23	87	49
Net (paid) recovered losses	(81)	(115)	(100)	(148)
Net expected loss to be paid (recovered), end of period	103	319	103	319	116
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	29	36	33	44	44
Total economic loss development (benefit)	1	(1)	(64)	0
Net (paid) recovered losses	30	(2)	91	(11)
Net expected loss to be paid (recovered), end of period	60	33	60	33	33
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	(6)	36	(10)	87	87
Total economic loss development (benefit)	(30)	(57)	(104)	(69)
Net (paid) recovered losses	37	8	115	(31)
Net expected loss to be paid (recovered), end of period	1	(13)	$ 1	(13)	$ (10)
Minimum
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Risk free discount rate			1.92%		1.98%
Maximum
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Risk free discount rate			5.70%		5.22%
Weighted Average
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Risk free discount rate			4.02%		4.38%
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	53	374	$ 18	398	$ 398
Total economic loss development (benefit)	(2)	(23)	51	(14)
Net (paid) recovered losses	(81)	(113)	(99)	(146)
Net expected loss to be paid (recovered), end of period	(30)	238	(30)	238	18
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	(35)	0	(43)	43	43
Total economic loss development (benefit)	(31)	(56)	(40)	(69)
Net (paid) recovered losses	7	10	24	(20)
Net expected loss to be paid (recovered), end of period	(59)	(46)	(59)	(46)	(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	139	37	98	20	20
Total economic loss development (benefit)	(6)	46	36	63
Net (paid) recovered losses	0	(2)	(1)	(2)
Net expected loss to be paid (recovered), end of period	$ 133	$ 81	$ 133	$ 81	$ 98