Expected Loss to be Paid - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) Transaction	Sep. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) scenario	Sep. 30, 2017 USD ($) Transaction Curve Payment scenario	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) scenario	Jan. 31, 2017 USD ($)	Jan. 10, 2017 USD ($)	Jul. 01, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 26, 2012 USD ($)	Feb. 05, 2009 USD ($)
Schedule of Expected Losses to be Paid [Line Items]
Period of insured credit performance of guaranteed obligations (in some cases over)				30 years
Discount factor (as a percent)				2.27%		2.73%
Net par amount outstanding	$ 275,767.0			$ 275,767.0		$ 296,318.0
Net expected loss to be paid after recoveries for R&W	1,292.0	$ 1,090.0	$ 1,297.0	1,292.0	$ 1,090.0	$ 1,198.0		$ 21.0	$ 22.0	$ 1,326.0	$ 1,391.0
Economic loss development after recoveries for R&W	204.0	(44.0)		$ 298.0	37.0
Minimum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Claim Liability, Risk Free Discount Rate				0.00%		0.00%
Maximum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Insured Financial Obligations, Claim Liability, Risk Free Discount Rate				2.94%		3.23%
Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	4,966.0			$ 4,966.0		$ 4,786.0
RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	5,064.0			5,064.0		5,637.0
Net expected loss to be paid after recoveries for R&W	176.0	148.0	182.0	176.0	148.0	206.0				192.0	409.0
Economic loss development after recoveries for R&W	(19.0)	(27.0)		(70.0)	(139.0)
Future net R&W benefit	$ 3.0			$ 3.0		6.0
RMBS [Member] | United States [Member] | Minimum [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	2.50%			2.50%
HELOCs [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Initial period for which borrower can pay only interest payments				10 years
Loan Modification, Extended Period for Which Borrower Can Pay Only Interest Payments				5 years
Trust Preferred Securities (TruPS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 1,455.0			$ 1,455.0		1,892.0
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	6,247.0			6,247.0		15,553.0
Net expected loss to be paid after recoveries for R&W	30.0	34.0	33.0	30.0	34.0	34.0		8.0	2.0	34.0	74.0
Economic loss development after recoveries for R&W	(5.0)	(2.0)		(3.0)	(19.0)
Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	2,058.0			2,058.0		2,057.0
Net expected loss to be paid after recoveries for R&W	(7.0)	54.0	(4.0)	(7.0)	54.0	54.0				100.0	99.0
Economic loss development after recoveries for R&W	(1.0)	(23.0)		(56.0)	(21.0)
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	1,400.0			1,400.0
Net expected loss to be paid after recoveries for R&W	34.0			34.0
Economic loss development after recoveries for R&W	1.0			2.0
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	260,943.0			260,943.0		271,179.0
Net expected loss to be paid after recoveries for R&W	1,093.0	854.0	1,086.0	1,093.0	854.0	904.0				1,000.0	809.0
Economic loss development after recoveries for R&W	229.0	8.0		427.0	216.0
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	218,216.0			218,216.0		244,798.0
Net expected loss to be paid after recoveries for R&W	1,046.0	816.0	1,044.0	1,046.0	816.0	871.0				963.0	771.0
Economic loss development after recoveries for R&W	229.0	9.0		431.0	218.0
Public Finance [Member] | City of Hartford, Connecticut [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	346.0			346.0
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	42,727.0			42,727.0		26,381.0
Net expected loss to be paid after recoveries for R&W	47.0	38.0	$ 42.0	47.0	38.0	33.0				$ 37.0	$ 38.0
Economic loss development after recoveries for R&W	0.0	(1.0)		(4.0)	(2.0)
Public Finance Stockton Pension Oblgiation Bonds [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	113.0			113.0
Non-Infrastructure Public Finance [Member] | Spain [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	456.0			456.0
Non-Infrastructure Public Finance [Member] | Portugal [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	75.0			75.0
BIG [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	13,226.0			13,226.0		13,074.0
BIG [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	5,000.0			5,000.0
BIG [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	2,869.0			2,869.0		3,151.0
BIG [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	239.0			239.0		430.0
BIG [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	418.0			418.0		645.0
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 85.0			$ 85.0		126.0
Insured financial obligations, transactions related BIG | Transaction	2			2
BIG [Member] | Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 116.0			$ 116.0
BIG [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	9,615.0			9,615.0		8,722.0
BIG [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	7,334.0			7,334.0		7,380.0
BIG [Member] | Public Finance [Member] | City of Hartford, Connecticut [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	345.0			345.0
BIG [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	2,281.0			2,281.0		$ 1,342.0
BIG [Member] | Non-Infrastructure Public Finance [Member] | Hungary [Member] | Sovereign and Sub Sovereign [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 214.0			$ 214.0
First Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Number of delinquent payments | Payment				2
Projected loss assumptions, CDR, plateau rate, projection period				36 months
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue				12 months
Intermediate conditional default rate (as a percent)	5.00%			5.00%
Number of scenarios weighted in estimating expected losses | scenario			5	5		5
First Lien [Member] | Base Scenario [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period				36 months
Period from plateau to intermediate conditional default rate (in months)				12 months
Period of constant intermediate conditional default rate (in months)				36 months
Intermediate conditional default rate as a percentage of plateau conditional default rate				20.00%
Final conditional default rate as a percentage of plateau conditional default rate				5.00%
Projected loss assumptions, Final CDR, Period for voluntary prepayments to continue				5 years 9 months
Default from delinquentor rate, term				36 months
Guarantor obligations, default period currently performing				36 months
Projected loss assumptions, loss severity, subsequent period				18 months
Estimated loss severity rate, one through six months (as a percent)				18 months
Loss severity (as a percent)	40.00%			40.00%
Projected loss assumptions, period to reach final loss severity rate				2 years 6 months
Final CPR	15.00%		15.00%	15.00%		15.00%
First Lien [Member] | More Stressful Environment [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)				15 months
Projected loss assumptions, period to reach final loss severity rate				9 years
First Lien [Member] | More Stressful Environment [Member] | RMBS [Member] | United States [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				$ 41.0
First Lien [Member] | More Stressful Environment [Member] | RMBS [Member] | United States [Member] | Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				1.0
First Lien [Member] | More Stressful Environment [Member] | RMBS [Member] | United States [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				9.0
First Lien [Member] | More Stressful Environment [Member] | RMBS [Member] | United States [Member] | Alt-A [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				$ 24.0
First Lien [Member] | Least Stressful Environment [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period				30 months
Period from plateau to intermediate conditional default rate (in months)				9 months
Decrease in the plateau period used to calculate potential change in loss estimate (in months)				6 months
First Lien [Member] | Least Stressful Environment [Member] | RMBS [Member] | United States [Member] | Subprime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				$ (22.0)
First Lien [Member] | Least Stressful Environment [Member] | RMBS [Member] | United States [Member] | Prime [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				(0.1)
First Lien [Member] | Least Stressful Environment [Member] | RMBS [Member] | United States [Member] | Option ARM [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				(21.0)
First Lien [Member] | Least Stressful Environment [Member] | RMBS [Member] | United States [Member] | Alt-A [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, increase (decrease) in expected loss to be paid, net				$ (11.0)
Second Lien [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)				28 months
Number of scenarios weighted in estimating expected losses | scenario				5		5
Period of loan default estimate				6 months
Number of preceding months average liquidation rates used to estimate loan default rate				6 months
Projected loss assumptions, period of consistent conditional default rate				6 months
Stress period (in months)				34 months
Loss recovery assumption (as a percent)	2.00%		2.00%	2.00%		2.00%
Number of conditional default rate curves modeled in estimating losses | Curve				5
Monthly delinquency threshold				6 months
Second Lien [Member] | RMBS [Member] | United States [Member] | Home Equity Line of Credit and Closed-end Mortgage [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	15.00%		15.00%	15.00%		15.00%
Second Lien [Member] | Base Scenario [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Period from plateau to intermediate conditional default rate (in months)				28 months
Stress period (in months)				34 months
Period of constant conditional default rate (in months)				6 months
Second Lien [Member] | Base Scenario One [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Projected loss assumptions, CDR, plateau rate, projection period				8 months
Period from plateau to intermediate conditional default rate (in months)				31 months
Stress period (in months)				39 months
Increase in conditional default rate ramp down period				3 months
Second Lien [Member] | Base Scenario One [Member] | RMBS [Member] | United States [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for increased conditional default rate plateau period				$ 13.0
Second Lien [Member] | Based Scenario Two [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Stress period (in months)				29 months
Period of constant conditional default rate (in months)				4 months
Change in estimate for decreased prepayment rate, Percent				10.00%
Decreased conditional default rate ramp down period				25 months
Second Lien [Member] | Based Scenario Two [Member] | RMBS [Member] | United States [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Change in estimate for decreased conditional default rate ramp down period				$ 14.0
Southern District of Mississippi Vs Madison County, Mississippi [Member] | Parkway East [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 19.0			$ 19.0
Payment time period on annual debt service				2 years
MBIA UK Insurance Limited [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	13,000.0			$ 13,000.0
Net expected loss to be paid on Radian Asset portfolio	0.0	0.0		21.0	0.0
MBIA UK Insurance Limited [Member] | Europe [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	218.0			218.0
MBIA UK Insurance Limited [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W								0.0
MBIA UK Insurance Limited [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W								0.0
MBIA UK Insurance Limited [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W								13.0
MBIA UK Insurance Limited [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W								0.0
MBIA UK Insurance Limited [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W							$ 13.0	$ 13.0
CIFG Holding Inc. [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on Radian Asset portfolio	0.0	22.0		0.0	22.0
CIFG Holding Inc. [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W									(22.0)
CIFG Holding Inc. [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W									0.0
CIFG Holding Inc. [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W									42.0
CIFG Holding Inc. [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W									40.0
CIFG Holding Inc. [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W									$ 2.0
Financial Guarantee Accounted for as Credit Derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W	(6.0)			(6.0)		$ 10.0
Economic loss development after recoveries for R&W	(1.0)	$ (6.0)		(24.0)	$ (23.0)
ACA 2005-2 Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Credit Default Swap [Member] | Financial Guarantee Accounted for as Credit Derivatives [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W												$ 400.0
Libertas II Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Credit Default Swap [Member] | Financial Guarantee Accounted for as Credit Derivatives [Member] | CIFG Holdings Inc. vs JP Morgan Securities LLC [Member] | RMBS [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for R&W												$ 325.0
Pending Litigation [Member] | CIFG Holding Inc. Vs. GreenPoint Mortgage Funding, Inc. [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 19.0			$ 19.0									$ 500.0