Financial Guaranty Insurance Contracts (Details 8) (USD $) In Millions, unless otherwise specified	9 Months Ended																																					3 Months Ended																		3 Months Ended			12 Months Ended		3 Months Ended				3 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Alt-A first lien		Jun. 30, 2012 Alt-A first lien		Dec. 31, 2011 Alt-A first lien		Sep. 30, 2012 Alt-A first lien High end of range		Jun. 30, 2012 Alt-A first lien High end of range		Dec. 31, 2011 Alt-A first lien High end of range		Sep. 30, 2012 Alt-A first lien Low end of range		Jun. 30, 2012 Alt-A first lien Low end of range		Dec. 31, 2011 Alt-A first lien Low 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] 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 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 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] 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		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 Total first lien scenario	Sep. 30, 2012 Prime first lien scenario	Sep. 30, 2012 Second lien	Dec. 31, 2011 Second lien	Sep. 30, 2012 RMBS	Sep. 30, 2012 Somewhat Stressful Environment [Member] Alt-A first lien	Sep. 30, 2012 Somewhat Stressful Environment [Member] First Mortgage Option Adjustable Rate Mortgage [Member]	Sep. 30, 2012 Somewhat Stressful Environment [Member] Residential, Subprime, Financing Receivable [Member]	Sep. 30, 2012 Somewhat Stressful Environment [Member] Total first lien	Sep. 30, 2012 Somewhat Stressful Environment [Member] Prime first lien	Sep. 30, 2012 More Stressful Environment [Member] Alt-A first lien	Sep. 30, 2012 More Stressful Environment [Member] First Mortgage Option Adjustable Rate Mortgage [Member]	Sep. 30, 2012 More Stressful Environment [Member] Residential, Subprime, Financing Receivable [Member]	Sep. 30, 2012 More Stressful Environment [Member] Prime first lien	Mar. 31, 2012 More Stressful Environment [Member] Prime first lien	Sep. 30, 2012 Somewhat Less Stressful Environment [Member] Alt-A first lien	Sep. 30, 2012 Somewhat Less Stressful Environment [Member] First Mortgage Option Adjustable Rate Mortgage [Member]	Sep. 30, 2012 Somewhat Less Stressful Environment [Member] Residential, Subprime, Financing Receivable [Member]	Sep. 30, 2012 Somewhat Less Stressful Environment [Member] Total first lien	Sep. 30, 2012 Somewhat Less Stressful Environment [Member] Prime first lien	Sep. 30, 2012 Least Stressful Environment [Member] Alt-A first lien	Sep. 30, 2012 Least Stressful Environment [Member] First Mortgage Option Adjustable Rate Mortgage [Member]	Sep. 30, 2012 Least Stressful Environment [Member] Residential, Subprime, Financing Receivable [Member]	Sep. 30, 2012 Least Stressful Environment [Member] Prime first lien	Mar. 31, 2012 Least Stressful Environment [Member] Prime first lien
Key Variables
Period until which loss severity rate would continue (in months)																																						6 months
Plateau conditional default rate (as a percent)								23.50%	[1]	23.00%	[1]	41.30%	[1]	3.50%	[1]	3.30%	[1]	2.80%	[1]							27.70%	[1]	30.20%	[1]	31.50%	[1]	7.60%	[1]	9.30%	[1]	11.70%	[1]							28.40%	[1]	29.20%	[1]	29.90%	[1]	7.50%	[1]	7.20%	[1]	8.60%	[1]
Intermediate conditional default rate (as a percent)								4.70%	[1]	4.60%	[1]	8.30%	[1]	0.70%	[1]	0.70%	[1]	0.60%	[1]							5.50%	[1]	6.00%	[1]	6.30%	[1]	1.50%	[1]	1.90%	[1]	2.30%	[1]							5.70%	[1]	5.80%	[1]	6.00%	[1]	1.50%	[1]	1.40%	[1]	1.70%	[1]					5.00%
Final conditional default rate trended down to (as a percent)								1.20%	[1]	1.20%	[1]	2.10%	[1]	0.20%	[1]	0.20%	[1]	0.10%	[1]							1.40%	[1]	1.50%	[1]	1.60%	[1]	0.40%	[1]	0.50%	[1]	0.60%	[1]							1.40%	[1]	1.50%	[1]	1.50%	[1]	0.40%	[1]	0.40%	[1]	0.40%	[1]
Loss severity (as a percent)		65.00%	[1]	65.00%	[1]	65.00%	[1]													65.00%	[1]	65.00%	[1]	65.00%	[1]													90.00%	[1]	90.00%	[1]	90.00%	[1]
Initial conditional prepayment rate (as a percent)								31.90%	[1]	27.10%	[1]	24.40%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]							4.00%	[1]	4.90%	[1]	10.80%	[1]	0.00%	[1]	0.60%	[1]	0.30%	[1]							7.40%	[1]	8.80%	[1]	16.30%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]
Final conditional prepayment rate (as a percent)	10.00%	15.00%	[1]	15.00%	[1]	15.00%	[1]													15.00%	[1]	15.00%	[1]	15.00%	[1]													15.00%	[1]	15.00%	[1]	15.00%	[1]
Period from initial to final conditional prepayment rate (in months)																																																								12 months																		12 months
Current conditional prepayment rate used in alternate scenario for loss estimate (as a percent)																																																								15.00%
Prior conditional prepayment rate used in alternate scenario for loss estimate (as a percent)																																																								10.00%
Increase in the plateau period used to calculate potential change in loss estimate (in months)																																																																	3 months
Increased plateau period used to calculate potential change in loss estimate (in months)																																																											1 month						27 months
Number of scenarios weighted in estimating expected losses																																																									5
Sensitivity Analysis, Number of Months Recovery Delayed Under Most Stressful Scenario	3 months
Sensitivity Analysis, Number of Months Recovery Expedited Under Least Stressful Scenario	3 months
Increased final loss severity for subprime transactions used to calculate potential change in loss estimate (as a percent)																																																																60.00%
Prior loss severity recovery period used to calculate potential change in loss estimate (in years)																																																																	4 years
Current loss severity recovery period used to calculate potential change in loss estimate (in years)																																																																	2 years									2 years
Loss severity recovery period used to calculate potential change in loss estimate in even more stressful scenario (in years)																																																								8 years
Conditional Default Rate Ramp Down Period	30 months																																																									30 months											15 months	12 months									9 months	12 months
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario																																																																		$ 71	$ 56	$ 167	$ 4
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period																																																													25	23	108		1
Initial subprime loss severity rate assumed to be recovered over two years (as a percent)																																																																										40.00%
Number of scenarios where the recovery was faster than in base case																																																								2
Initial subprime loss severity rate assumed for 12 months (as a percent)																																																																										80.00%
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate																																																																							4	30	29		0
Decrease in the plateau period used to calculate potential change in loss estimate (in months)																																																								3 months
Decreased plateau period used to calculate potential change in loss estimate (in months)																																																								21 months
Decrease in expected loss in case of decrease in conditional default rate plateau period																																																																												$ 28	$ 68	$ 56	$ 1

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