Financial Guaranty Insurance Contracts (Details 6) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended																			3 Months Ended						3 Months Ended																				3 Months Ended														3 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended	3 Months Ended
	Sep. 30, 2012 contract	Sep. 30, 2012 contract	Sep. 30, 2012 Alt-A First Mortgage [Member]		Jun. 30, 2012 Alt-A First Mortgage [Member]		Dec. 31, 2011 Alt-A First Mortgage [Member]		Sep. 30, 2012 Alt-A First Mortgage [Member] Low end of range		Jun. 30, 2012 Alt-A First Mortgage [Member] Low end of range		Dec. 31, 2011 Alt-A First Mortgage [Member] Low end of range		Sep. 30, 2012 Alt-A First Mortgage [Member] High end of range		Jun. 30, 2012 Alt-A First Mortgage [Member] High end of range		Dec. 31, 2011 Alt-A First Mortgage [Member] High end of range		Sep. 30, 2012 HELOCs		Jun. 30, 2012 HELOCs		Dec. 31, 2011 HELOCs		Sep. 30, 2012 HELOCs Scenario 1 [Member]	Sep. 30, 2012 HELOCs Scenario 2 [Member]	Sep. 30, 2012 HELOCs Low end of range		Jun. 30, 2012 HELOCs Low end of range		Dec. 31, 2011 HELOCs Low end of range		Sep. 30, 2012 HELOCs High end of range		Jun. 30, 2012 HELOCs High end of range		Dec. 31, 2011 HELOCs High end of range		Sep. 30, 2012 CES		Jun. 30, 2012 CES		Dec. 31, 2011 CES		Sep. 30, 2012 CES Scenario 1 [Member]	Sep. 30, 2012 CES Scenario 2 [Member]	Sep. 30, 2012 CES Low end of range		Jun. 30, 2012 CES Low end of range		Dec. 31, 2011 CES Low end of range		Sep. 30, 2012 CES High end of range		Jun. 30, 2012 CES High end of range		Dec. 31, 2011 CES High end of range		Sep. 30, 2012 Second lien Curve	Dec. 31, 2011 Second lien	Jan. 31, 2012 Second lien Base Scenario	Sep. 30, 2012 Second lien Base Scenario	Sep. 30, 2012 Second lien Base Scenario	Dec. 31, 2011 Second lien Base Scenario	Jan. 31, 2012 Second lien Scenario 1 [Member]	Sep. 30, 2012 Second lien Scenario 1 [Member]	Mar. 31, 2012 Second lien Scenario 1 [Member]	Dec. 31, 2011 Second lien Scenario 1 [Member]	Jan. 31, 2012 Second lien Scenario 2 [Member]	Sep. 30, 2012 Second lien Scenario 2 [Member]	Mar. 31, 2012 Second lien Scenario 2 [Member]	Dec. 31, 2011 Second lien Scenario 2 [Member]	Sep. 30, 2012 Second lien Low end of range	Sep. 30, 2012 Second lien High end of range	Sep. 30, 2012 First Mortgage Option Adjustable Rate Mortgage [Member]		Jun. 30, 2012 First Mortgage Option Adjustable Rate Mortgage [Member]		Dec. 31, 2011 First Mortgage Option Adjustable Rate Mortgage [Member]		Sep. 30, 2012 First Mortgage Option Adjustable Rate Mortgage [Member] Low end of range		Jun. 30, 2012 First Mortgage Option Adjustable Rate Mortgage [Member] Low end of range		Dec. 31, 2011 First Mortgage Option Adjustable Rate Mortgage [Member] Low end of range		Sep. 30, 2012 First Mortgage Option Adjustable Rate Mortgage [Member] High end of range		Jun. 30, 2012 First Mortgage Option Adjustable Rate Mortgage [Member] High end of range		Dec. 31, 2011 First Mortgage Option Adjustable Rate Mortgage [Member] High end of range		Sep. 30, 2012 Residential, Subprime, Financing Receivable [Member]		Jun. 30, 2012 Residential, Subprime, Financing Receivable [Member]		Dec. 31, 2011 Residential, Subprime, Financing Receivable [Member]		Sep. 30, 2012 Residential, Subprime, Financing Receivable [Member] Low end of range		Jun. 30, 2012 Residential, Subprime, Financing Receivable [Member] Low end of range		Dec. 31, 2011 Residential, Subprime, Financing Receivable [Member] Low end of range		Sep. 30, 2012 Residential, Subprime, Financing Receivable [Member] High end of range		Jun. 30, 2012 Residential, Subprime, Financing Receivable [Member] High end of range		Dec. 31, 2011 Residential, Subprime, Financing Receivable [Member] High end of range
Key Variables
Period Company Reduced Most Optimistic Case for Market Recovery	3 months
Plateau conditional default rate (as a percent)									3.50%	[1]	3.30%	[1]	2.80%	[1]	23.50%	[1]	23.00%	[1]	41.30%	[1]									2.20%	[1]	2.90%	[1]	4.00%	[1]	12.90%	[1]	20.90%	[1]	27.40%	[1]									6.20%	[1]	4.30%	[1]	6.90%	[1]	19.00%	[1]	20.70%	[1]	24.80%	[1]																							7.60%	[1]	9.30%	[1]	11.70%	[1]	27.70%	[1]	30.20%	[1]	31.50%	[1]							7.50%	[1]	7.20%	[1]	8.60%	[1]	28.40%	[1]	29.20%	[1]	29.90%	[1]
Intermediate Conditional Default Rate									0.70%	[1]	0.70%	[1]	0.60%	[1]	4.70%	[1]	4.60%	[1]	8.30%	[1]																																																															1.50%	[1]	1.90%	[1]	2.30%	[1]	5.50%	[1]	6.00%	[1]	6.30%	[1]							1.50%	[1]	1.40%	[1]	1.70%	[1]	5.70%	[1]	5.80%	[1]	6.00%	[1]
Final conditional default rate trended down to (as a percent)									0.20%	[1]	0.20%	[1]	0.10%	[1]	1.20%	[1]	1.20%	[1]	2.10%	[1]									0.40%	[1]	0.40%	[1]	0.40%	[1]	3.20%	[1]	3.20%	[1]	3.20%	[1]									3.30%	[1]	3.30%	[1]	3.50%	[1]	9.10%	[1]	9.10%	[1]	9.20%	[1]																							0.40%	[1]	0.50%	[1]	0.60%	[1]	1.40%	[1]	1.50%	[1]	1.60%	[1]							0.40%	[1]	0.40%	[1]	0.40%	[1]	1.40%	[1]	1.50%	[1]	1.50%	[1]
Expected period until final conditional default rate (in months)																					36 months		36 months	[1]	36 months	[1]															36 months	[1]	36 months	[1]	36 months	[1]
Initial conditional prepayment rate (as a percent)									0.00%	[1]	0.00%	[1]	0.00%	[1]	31.90%	[1]	27.10%	[1]	24.40%	[1]									3.70%	[1]	2.70%	[1]	1.40%	[1]	17.50%	[1]	16.40%	[1]	25.80%	[1]									1.80%	[1]	1.10%	[1]	0.90%	[1]	10.10%	[1]	11.00%	[1]	14.70%	[1]																							0.00%	[1]	0.60%	[1]	0.30%	[1]	4.00%	[1]	4.90%	[1]	10.80%	[1]							0.00%	[1]	0.00%	[1]	0.00%	[1]	7.40%	[1]	8.80%	[1]	16.30%	[1]
Final conditional prepayment rate (as a percent)	10.00%	10.00%	15.00%	[1]	15.00%	[1]	15.00%	[1]													10.00%	[1]	10.00%	[1]	10.00%	[1]															10.00%	[1]	10.00%	[1]	10.00%	[1]																															15.00%	[1]	15.00%	[1]	15.00%	[1]													15.00%	[1]	15.00%	[1]	15.00%	[1]
Loss severity (as a percent)			65.00%	[1]	65.00%	[1]	65.00%	[1]													98.00%	[1]	98.00%	[1]	98.00%	[1]															98.00%	[1]	98.00%	[1]	98.00%	[1]																															65.00%	[1]	65.00%	[1]	65.00%	[1]													90.00%	[1]	90.00%	[1]	90.00%	[1]
Initial draw rate (as a percent)																													0.00%	[1]	0.00%	[1]	0.00%	[1]	3.50%	[1]	4.10%	[1]	15.30%	[1]
Typical past due period for loans to be charged off (in days)																																																													180 days
Number of Monthly Delinquency Categories		5 months
First delinquency category (in days)																																																																											30 days	59 days
Second delinquency category (in days)																																																																											60 days	89 days
Third delinquency category (in days)																																																																											90 days	119 days
Fourth delinquency category (in days)																																																																											120 days	149 days
Fifth delinquency category (in days)																																																																											150 days	179 days
Period of Default Estimate																																																													5 months
Number of Preceding Months Liquidation Rates Used to Estimate Loan Default Rate																																																													12 months
Number of Months the Conditional Default Rate is Calculated Applying Liquidation Rates to Current Period Past Due Balances Excluding Thirty to Fifty Nine Days Past Due																																																													4 months
Period of constant conditional default rate (in months)		1 month																																																													1 month	1 month		1 month						1 month
Period of decrease in conditional default rate until final rate is achieved (in months)		30 months																																																											30 months		30 months	30 months		30 months		33 months	30 months	30 months				24 months
Stress period (in months)		36 months																																																													36 months		36 months	36 months		42 months	39 months	39 months		27	30 months	30 months
Length of Change of Ramp Downs in Both Scenarios in Sensitivity Analysis	3 months
Loss Recovery Assumption	2.00%	2.00%
Conditional Prepayment Rate Base Case Average Number of Months		3 months
Period from Initial to Final Draw Rate																					3 months
Final Draw Rate																													0.00%						1.80%
Number of conditional default rate curves modeled in estimating losses																																																													3
Increased plateau period used to calculate potential change in loss estimate (in months)																																																														1 month					4 months	4 months		4 months	1 month
Loss Estimate Sensitivity Analysis Increase in Conditional Default Rate Ramp Down Period		3 months
Loss Estimate Sensitivity Analysis Change in Estimate for Increased Conditional Default Rate Plateau Period																											$ 53
Decreased ramp down period used to calculate potential change in loss estimate (in months)																																																																								21 months	24 months
Loss Estimate Sensitivity Analysis Change in Estimate for Decreased Conditional Default Rate Ramp Down Period																												$ 52																			$ 4	$ 3
Period Company Extended Most Pessimistic Case for Market Recovery	3 months
Number of Types of Second Lien Residential Mortgage Back Securities Insured	2	2

[1] Represents variables for most heavily weighted scenario (the “base case”).