Financial Guaranty Contracts Accounted for as Insurance (Details 5) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended		6 Months Ended			6 Months Ended
	Jun. 30, 2011 Second lien M D RatingCategories RateCurves Transactions Securities Loans	Dec. 31, 2010 Second lien M	Dec. 31, 2009 Second lien M	Jun. 30, 2011 CES Policies M	Mar. 31, 2011 CES M	Dec. 31, 2010 CES M Policies	Jun. 30, 2011 HELOCs M Policies	Mar. 31, 2011 HELOCs M	Dec. 31, 2010 HELOCs Policies M
Key Variables
Plateau conditional default rate, low end of range (as a percent)				4.80%	7.20%	7.30%	4.60%	4.70%	4.20%
Plateau conditional default rate, high end of range (as a percent)				22.80%	28.90%	27.10%	34.60%	21.40%	22.10%
Final conditional default rate trended down to, low end of range (as a percent)				2.90%	2.90%	2.90%	0.40%	0.40%	0.40%
Final conditional default rate trended down to, high end of range (as a percent)				8.10%	8.10%	8.10%	3.20%	3.20%	3.20%
Expected period until final conditional default rate (in months)				36	36	24	36	36	24
Initial conditional prepayment rate low end of range (as a percent)				1.40%	0.90%	1.30%	0.90%	0.90%	3.30%
Initial conditional prepayment rate, high end of range (as a percent)				12.00%	12.70%	9.70%	15.50%	12.60%	17.50%
Final conditional prepayment rate (as a percent)				10.00%	10.00%	10.00%	10.00%	10.00%	10.00%
Loss severity (as a percent)				98.00%	98.00%	98.00%	98.00%	98.00%	98.00%
Initial draw rate, low end of range (as a percent)							0.00%	0.00%	0.00%
Initial draw rate, high end of range (as a percent)							8.60%	5.20%	6.80%
Typical past due period for loans to be charged off (in days)	180
Number of monthly delinquency categories	5
First delinquency category, low end of range (in days)	30
First delinquency category, high end of range (in days)	59
Second delinquency category, low end of range (in days)	60
Second delinquency category, high end of range (in days)	89
Third delinquency category, low end of range (in days)	90
Third delinquency category, high end of range (in days)	119
Fourth delinquency category, low end of range (in days)	120
Fourth delinquency category, high end of range (in days)	149
Fifth delinquency category, low end of range (in days)	150
Fifth delinquency category, high end of range (in days)	179
Period of default estimate (in months)	5
Lookback period for liquidation rates used to estimate default (in months)	12
Period for which default rate is calculated based on current period past due balances (in months)	4
Lookback period for past due balances used to estimate default in fifth month (in months)	3
Period of constant conditional default rate (in months)	1
Period of decrease in conditional default rate until final rate is achieved (in months)	30	18
Stress period (in months)	36		30
Change in stress period (in months)	12	24
Loss recovery assumption (as a percent)	2.00%
Period from initial to final draw rate (in months)							3
Final draw rate, low end of range (as a percent)							0.00%
Final draw rate, high end of range (as a percent)							4.30%
Number of conditional default rate curves modeled in estimating losses	3
Increased plateau period used to calculate potential change in loss estimate (in months)	4
Increase in expected loss in case of increase in conditional default rate plateau period				$ 7.9			$ 72.1
Decreased ramp down period used to calculate potential change in loss estimate (in months)	24
Decrease in expected loss in case of decrease in conditional default rate ramp down period				$ 3.0			$ 66.7