Expected Loss to be Paid - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Curve Payment scenario	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) scenario	Dec. 31, 2016 USD ($)	Nov. 26, 2012 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)	2.82%		2.38%
Net par amount outstanding	$ 257,089		$ 264,952
Net expected loss to be paid after recoveries for R&W	1,298	$ 1,244	$ 1,303	$ 1,198
Net expected loss to be paid on Radian Asset portfolio		21
Economic loss development after recoveries for R&W	$ 24	(47)
Minimum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate	0.00%		0.00%
Maximum [Member]
Schedule of Expected Losses to be Paid [Line Items]
Risk free discount rate	3.11%		2.78%
Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 4,967		$ 4,966
Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid after recoveries for representations and warranties, percent	3.50%		3.70%
RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	$ 4,678		$ 4,818
Net expected loss to be paid after recoveries for R&W	219	197	73	206
Net expected loss to be paid on Radian Asset portfolio		0
Economic loss development after recoveries for R&W	(16)	22
Future net R&W benefit	$ 19		117
RMBS [Member] | United States [Member] | Minimum [Member] | HELOCs [Member]
Schedule of Expected Losses to be Paid [Line Items]
Final CPR	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
Extended period for which borrow 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	$ 1,188		1,349
Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	4,945		5,272
Net expected loss to be paid after recoveries for R&W	29	36	27	88
Net expected loss to be paid on Radian Asset portfolio		8
Economic loss development after recoveries for R&W	(2)	50
Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	1,194		1,199
Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	1,200
Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	245,084		252,314
Net expected loss to be paid after recoveries for R&W	1,050	1,011	1,203	904
Net expected loss to be paid on Radian Asset portfolio		13
Economic loss development after recoveries for R&W	42	(119)
Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	201,337		209,392
Net expected loss to be paid after recoveries for R&W	1,007	970	1,157	871
Net expected loss to be paid on Radian Asset portfolio		0
Economic loss development after recoveries for R&W	39	(124)
Public Finance [Member] | City of Hartford, Connecticut [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	341
Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	43,747		42,922
Net expected loss to be paid after recoveries for R&W	43	41	46	$ 33
Net expected loss to be paid on Radian Asset portfolio		13
Economic loss development after recoveries for R&W	3	5
Public Finance Stockton Pension Oblgiation Bonds [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	113
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	470
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	73
BIG [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	11,599		12,238
BIG [Member] | Puerto Rico [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	5,000
BIG [Member] | RMBS [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	2,666		2,761
BIG [Member] | Trust Preferred Securities (TruPS) [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	99		161
BIG [Member] | Other structured finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	348		360
BIG [Member] | Triple-X Life Insurance Transaction [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	85		85
BIG [Member] | Student Loan [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	113
BIG [Member] | Public Finance [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	8,401		8,871
BIG [Member] | Public Finance [Member] | United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	6,646		7,140
BIG [Member] | Public Finance [Member] | Non United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	1,755		1,731
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	207
First Lien [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	152		$ 123
Economic loss development after recoveries for R&W	$ 24	9
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%
Number of scenarios weighted in estimating expected losses | scenario	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 3 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%
Projected loss assumptions, period to reach final loss severity rate	2 years 6 months
Final CPR	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
Projected loss assumptions, increase (decrease) in expected loss to be paid, net	$ 65
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
Projected loss assumptions, increase (decrease) in expected loss to be paid, net	$ 41
Decrease in the plateau period used to calculate potential change in loss estimate (in months)	6 months
Second Lien [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	$ 67		$ 50
Economic loss development after recoveries for R&W	$ 8	13
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%
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%
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	$ 11
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	$ 12
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
Payment time period on annual debt service	2 years
MBIA UK Insurance Limited [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid on Radian Asset portfolio	$ 0	21
MBIA UK Insurance Limited [Member] | Europe [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net par amount outstanding	223
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)		$ (14)
Economic loss development after recoveries for R&W	$ (7)	$ 15
ACA 2005-2 Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Total [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
Libertas II Collateralized Debt Obligations [Member] | CIFG Holding Inc. [Member] | Total [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