Expected Loss to be Paid - Net Expected Loss to be Paid After Recoveries for Breaches of R&W (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	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 of R&W	$ 1,198	$ 1,391
Net expected loss to be paid	21	22
Total economic loss development (benefit)	313	139
Accretion of discount	33	26
Changes in discount rates	25	(15)
Changes in timing and assumptions	255	128
(Paid) Recovered Losses After Recoveries for R&W	(229)	(354)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W	$ 1,303	1,198
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	$ 24	16
Expected LAE to be paid	23	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 of R&W	904	809
Net expected loss to be paid	13	42
Total economic loss development (benefit)	549	269
(Paid) Recovered Losses After Recoveries for R&W	(263)	(216)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W	1,203	904
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 of R&W	871	771
Net expected loss to be paid	0	40
Total economic loss development (benefit)	554	276
(Paid) Recovered Losses After Recoveries for R&W	(268)	(216)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W	1,157	871
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 of R&W	33	38
Net expected loss to be paid	13	2
Total economic loss development (benefit)	(5)	(7)
(Paid) Recovered Losses After Recoveries for R&W	5	0
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W	46	33
Residential Mortgage-Backed Securities (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 of R&W	206	409
Net expected loss to be paid	0	(22)
Total economic loss development (benefit)	(181)	(91)
(Paid) Recovered Losses After Recoveries for R&W	48	(90)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W	73	206
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 of R&W	88	173
Net expected loss to be paid	8	2
Total economic loss development (benefit)	(55)	(39)
(Paid) Recovered Losses After Recoveries for R&W	(14)	(48)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W	27	88
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 of R&W	294	582
Net expected loss to be paid	8	(20)
Total economic loss development (benefit)	(236)	(130)
(Paid) Recovered Losses After Recoveries for R&W	34	(138)
End of Period, Net Expected Loss To Be Paid After Recoveries of R&W	100	294
MBIA UK [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid	21	0
CIFG Holding Inc. [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid	$ 0	$ 22