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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
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	$ 1,298	$ 1,244	$ 1,303	$ 1,198
Economic Loss Development / (Benefit)	19	47	(5)	94
Accretion of discount	9	8	17	16
Changes in discount rates	0	23	(6)	34
Changes in timing and assumptions	10	16	(16)	44
(Paid) Recovered Losses	(16)	6	3	(16)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	1,432	1,297	$ 1,432	1,297
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	6	7	$ 11	9
Expected LAE to be paid	17		17		$ 23
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	1,050	1,011	1,203	904
Economic Loss Development / (Benefit)	53	79	11	198
(Paid) Recovered Losses	(22)	(4)	(133)	(29)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	1,082	1,086	1,082	1,086
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	1,007	970	1,157	871
Economic Loss Development / (Benefit)	56	78	17	202
(Paid) Recovered Losses	(22)	(4)	(133)	(29)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	1,041	1,044	1,041	1,044
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	43	41	46	33
Economic Loss Development / (Benefit)	(3)	1	(6)	(4)
(Paid) Recovered Losses	0	0	0	0
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	41	42	41	42
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	219	197	73	206
Economic Loss Development / (Benefit)	(28)	(29)	(12)	(51)
(Paid) Recovered Losses	5	14	135	27
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	326	182	326	182
RMBS [Member] | First Lien [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			123
Economic Loss Development / (Benefit)	7	14	(17)	23
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	248		248
RMBS [Member] | Second Lien [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			50
Economic Loss Development / (Benefit)	21	15	29	28
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	78		78
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	29	36	27	88
Economic Loss Development / (Benefit)	(6)	(3)	(4)	(53)
(Paid) Recovered Losses	1	(4)	1	(14)
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	24	29	24	29
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	248	233	100	294
Economic Loss Development / (Benefit)	(34)	(32)	(16)	(104)
(Paid) Recovered Losses	6	10	136	13
End of Period, Net Expected Loss to be Paid After Recoveries for R&W	350	211	350	211
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 acquired portfolio activity	0	0	0	21
MBIA UK Insurance Limited [Member] | Public Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity				13
MBIA UK Insurance Limited [Member] | Public Finance [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity				0
MBIA UK Insurance Limited [Member] | Public Finance [Member] | Non United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity				13
MBIA UK Insurance Limited [Member] | RMBS [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity				0
MBIA UK Insurance Limited [Member] | Other structured finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity				8
MBIA UK Insurance Limited [Member] | Structured Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on acquired portfolio activity				8
Reinsurance of SGI Insured Portfolio [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	131		131
Reinsurance of SGI Insured Portfolio [Member] | Public Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	1		1
Reinsurance of SGI Insured Portfolio [Member] | Public Finance [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	0		0
Reinsurance of SGI Insured Portfolio [Member] | Public Finance [Member] | Non United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	1		1
Reinsurance of SGI Insured Portfolio [Member] | RMBS [Member] | United States [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	130		130
Reinsurance of SGI Insured Portfolio [Member] | Other structured finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	0		0
Reinsurance of SGI Insured Portfolio [Member] | Structured Finance [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	130		130
AGC [Member] | Reinsurance of SGI Insured Portfolio [Member]
Present Value of Net Expected Loss and Loss Adjustment Expenses to be Paid [Roll Forward]
Net expected loss to be paid on assumed portfolio activity	$ 131	$ 0	$ 131	$ 0