Financial Guaranty Insurance Contracts (Details 8) (USD $) In Millions, unless otherwise specified	3 Months Ended								3 Months Ended								3 Months Ended								3 Months Ended
	Mar. 31, 2012 Alt-A first lien	Dec. 31, 2011 Alt-A first lien	Mar. 31, 2012 Alt-A first lien High end of range	Dec. 31, 2011 Alt-A first lien High end of range	Mar. 31, 2011 Alt-A first lien High end of range	Mar. 31, 2012 Alt-A first lien Low end of range	Dec. 31, 2011 Alt-A first lien Low end of range	Mar. 31, 2011 Alt-A first lien Low end of range	Mar. 31, 2012 Option ARM	Dec. 31, 2011 Option ARM	Mar. 31, 2012 Option ARM High end of range	Dec. 31, 2011 Option ARM High end of range	Mar. 31, 2011 Option ARM High end of range	Mar. 31, 2012 Option ARM Low end of range	Dec. 31, 2011 Option ARM Low end of range	Mar. 31, 2011 Option ARM Low end of range	Mar. 31, 2012 Subprime	Dec. 31, 2011 Subprime	Mar. 31, 2012 Subprime High end of range	Dec. 31, 2011 Subprime High end of range	Mar. 31, 2011 Subprime High end of range	Mar. 31, 2012 Subprime Low end of range	Dec. 31, 2011 Subprime Low end of range	Mar. 31, 2011 Subprime Low end of range	Mar. 31, 2012 Total first lien scenario	Mar. 31, 2012 Prime first lien
Key Variables
Period until which loss severity rate would continue (in months)																	6 months
Plateau conditional default rate (as a percent)			33.90%	41.30%		2.70%	2.80%				32.20%	31.50%		9.70%	11.70%				30.00%	29.90%		8.30%	8.60%
Intermediate conditional default rate (as a percent)			6.80%	8.30%	6.00%	0.50%	0.60%	0.40%			6.40%	6.30%	5.00%	1.90%	2.30%	1.80%			6.00%	6.00%	5.10%	1.70%	1.70%	1.20%
Final conditional default rate trended down to (as a percent)			1.70%	2.10%		0.10%	0.10%				1.60%	1.60%		0.50%	0.60%				1.50%	1.50%		0.40%	0.40%
Loss severity (as a percent)	65.00%	65.00%							65.00%	65.00%							90.00%	90.00%	90.00%			80.00%
Initial conditional prepayment rate (as a percent)			34.10%	24.40%		0.00%	0.00%				5.30%	10.80%		0.10%	0.30%				8.80%	16.30%		0.00%	0.00%
Final conditional prepayment rate (as a percent)	15.00%	15.00%							15.00%	15.00%							15.00%	15.00%
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
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
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	$ 26.7								$ 31.0								$ 120.0									$ 0.7
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%
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	67.8								65.9								167.1									2.3
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 expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	5.2								30.1								22.1									0.2
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	$ 24.4								$ 67.7								$ 46.3									$ 0.6