Expected Loss to be Paid - Net Expected Loss to be Paid After Net Expected Recoveries for Breaches of R&W Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 1,198	$ 1,391
Net expected loss to be paid on acquisitions	21
Economic loss development after recoveries for R&W	47	59
Accretion of discount	8	9
Changes in discount rates	11	63
Changes in timing and assumptions	28	(13)
(Paid) Recovered Losses After Recoveries for R&W	(22)	(113)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	$ 1,244	1,337
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	$ 2	2
Expected LAE to be paid	19		$ 12
Public Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	904	809
Net expected loss to be paid on acquisitions	13
Economic loss development after recoveries for R&W	119	99
(Paid) Recovered Losses After Recoveries for R&W	(25)	(5)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	1,011	903
Public Finance [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	871	771
Net expected loss to be paid on acquisitions	0
Economic loss development after recoveries for R&W	124	98
(Paid) Recovered Losses After Recoveries for R&W	(25)	(5)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	970	864
Public Finance [Member] | Non United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	33	38
Net expected loss to be paid on acquisitions	13
Economic loss development after recoveries for R&W	(5)	1
(Paid) Recovered Losses After Recoveries for R&W	0	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	41	39
RMBS [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	206	409
Net expected loss to be paid on acquisitions	0
Economic loss development after recoveries for R&W	(22)	(31)
(Paid) Recovered Losses After Recoveries for R&W	13	(85)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	197	293
Triple-X Life Insurance Transaction [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	54	99
Net expected loss to be paid on acquisitions	0
Economic loss development after recoveries for R&W	(53)	4
(Paid) Recovered Losses After Recoveries for R&W	0	(1)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	1	102
Student Loan [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Economic loss development after recoveries for R&W	2
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	34
Other structured finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	34	74
Net expected loss to be paid on acquisitions	8
Economic loss development after recoveries for R&W	3	(13)
(Paid) Recovered Losses After Recoveries for R&W	(10)	(22)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	35	39
Structured Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Beginning of Period, Net Expected Loss to be Paid After Recoveries for R&W	294	582
Net expected loss to be paid on acquisitions	8
Economic loss development after recoveries for R&W	(72)	(40)
(Paid) Recovered Losses After Recoveries for R&W	3	(108)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	233	434
MBIA UK Insurance Limited [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquisitions	$ 21	$ 0