Financial Guaranty Insurance Contracts (Details 8) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Key Variables
Loss severity (as a percent)		80.00%	70.00%
Total first lien
Key Variables
Period from initial to final conditional prepayment rate (in 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)	27 months
Number of scenarios weighted in estimating expected losses	5
Number of scenarios weighted more than prior periods in estimating expected losses in current period	1
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
Loss severity recovery period used to calculate potential change in loss estimate in even more stressful scenario (in years)	8 years
Final loss severity for subprime transactions used to calculate potential change in loss estimate in even more stressful scenario (as a percent)	60.00%
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%
Initial subprime loss severity rate assumed to be recovered over two years (as a percent)	40.00%
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
Prime first lien
Key Variables
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	$ 0.8
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	1.9
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	0.2
Decrease in expected loss in case of decrease in conditional default rate plateau period	0.6
Alt-A first lien
Key Variables
Loss severity (as a percent)	65.00%	60.00%	60.00%
Final conditional prepayment rate (as a percent)	15.00%	10.00%	10.00%
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	28.6
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	73.0
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	6.4
Decrease in expected loss in case of decrease in conditional default rate plateau period	27.3
Alt-A first lien | High end of range
Key Variables
Plateau conditional default rate (as a percent)	41.30%	42.20%	35.70%
Intermediate conditional default rate (as a percent)	8.30%	6.30%	5.40%
Final conditional default rate trended down to (as a percent)	2.10%	2.10%	1.80%
Initial conditional prepayment rate (as a percent)	24.40%	36.50%	20.50%
Alt-A first lien | Low end of range
Key Variables
Plateau conditional default rate (as a percent)	2.80%	2.60%	1.50%
Intermediate conditional default rate (as a percent)	0.60%	0.40%	0.20%
Final conditional default rate trended down to (as a percent)	0.10%	0.10%	0.10%
Initial conditional prepayment rate (as a percent)	0.00%	0.00%	0.00%
Option ARM
Key Variables
Loss severity (as a percent)	65.00%	60.00%	60.00%
Final conditional prepayment rate (as a percent)	15.00%	10.00%	10.00%
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	66.3
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	154.0
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	54.5
Decrease in expected loss in case of decrease in conditional default rate plateau period	120.0
Option ARM | High end of range
Key Variables
Plateau conditional default rate (as a percent)	31.50%	32.70%	27.00%
Intermediate conditional default rate (as a percent)	6.30%	4.90%	4.10%
Final conditional default rate trended down to (as a percent)	1.60%	1.60%	1.40%
Initial conditional prepayment rate (as a percent)	10.80%	17.70%	3.50%
Option ARM | Low end of range
Key Variables
Plateau conditional default rate (as a percent)	11.70%	11.70%	13.50%
Intermediate conditional default rate (as a percent)	2.30%	1.80%	2.00%
Final conditional default rate trended down to (as a percent)	0.60%	0.60%	0.70%
Initial conditional prepayment rate (as a percent)	0.30%	0.00%	0.00%
Subprime
Key Variables
Loss severity (as a percent)	90.00%	80.00%	70.00%
Final conditional prepayment rate (as a percent)	15.00%	10.00%	10.00%
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	111.3
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	160.1
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	24.3
Decrease in expected loss in case of decrease in conditional default rate plateau period	$ 51.9
Subprime | High end of range
Key Variables
Plateau conditional default rate (as a percent)	29.90%	34.60%	29.50%
Intermediate conditional default rate (as a percent)	6.00%	5.20%	4.40%
Final conditional default rate trended down to (as a percent)	1.50%	1.70%	1.50%
Loss severity (as a percent)	90.00%
Initial conditional prepayment rate (as a percent)	16.30%	13.50%	12.00%
Subprime | Low end of range
Key Variables
Plateau conditional default rate (as a percent)	8.60%	9.00%	7.10%
Intermediate conditional default rate (as a percent)	1.70%	1.30%	1.10%
Final conditional default rate trended down to (as a percent)	0.40%	0.40%	0.40%
Loss severity (as a percent)	80.00%
Initial conditional prepayment rate (as a percent)	0.00%	0.00%	0.00%