Financial Guaranty Insurance Contracts (Details 6) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011 category Curve	Dec. 31, 2010	Dec. 31, 2009
Key Variables
Loss severity (as a percent)			80.00%	70.00%
Second lien
Key Variables
Loss severity (as a percent)			98.00%	95.00%
Typical past due period for loans to be charged off (in days)	180 days	180 days
Number of monthly delinquency categories		5
Period of default estimate (in months)		5 months
Lookback period for liquidation rates used to estimate default (in months)		12 months
Period for which default rate is calculated based on current period past due balances (in months)		4 months
Lookback period for past due balances used to estimate default in fifth month (in months)		3 months
Period of constant conditional default rate (in months)		1 month
Period of decrease in conditional default rate until final rate is achieved (in months)	30 months	30 months	18 months	18 months
Stress period (in months)		36 months	36 months	24 months
Change in stress period (in months)	12 months		24 months
Loss recovery assumption (as a percent)			2.00%
Number of conditional default rate curves modeled in estimating losses		3
Increased plateau period used to calculate potential change in loss estimate (in months)		30 months
Total stress period in scenario with increased plateau period (in months)		39 months
Decreased ramp down period used to calculate potential change in loss estimate (in months)		24 months
Total stress period in scenario with decreased ramp down period (in months)		30 months
Second lien | Low end of range
Key Variables
First delinquency category (in days)		30 days
Second delinquency category (in days)		60 days
Third delinquency category (in days)		90 days
Fourth delinquency category (in days)		120 days
Fifth delinquency category (in days)		150 days
Second lien | High end of range
Key Variables
First delinquency category (in days)		59 days
Second delinquency category (in days)		89 days
Third delinquency category (in days)		119 days
Fourth delinquency category (in days)		149 days
Fifth delinquency category (in days)		179 days
Loss recovery assumption (as a percent)				5.00%
CES
Key Variables
Expected period until final conditional default rate (in months)	36 months	36 months	24 months	21 months
Final conditional prepayment rate (as a percent)	10.00%	10.00%	10.00%	10.00%
Loss severity (as a percent)	98.00%	98.00%	98.00%	95.00%
Increase in expected loss in case of increase in conditional default rate plateau period		5.0
Decrease in expected loss in case of decrease in conditional default rate ramp down period		2.9
CES | Low end of range
Key Variables
Plateau conditional default rate (as a percent)	6.90%	6.90%	7.30%	21.50%
Final conditional default rate trended down to (as a percent)	3.50%	3.50%	2.90%	3.30%
Initial conditional prepayment rate (as a percent)	0.90%	0.90%	1.30%	0.80%
CES | High end of range
Key Variables
Plateau conditional default rate (as a percent)	24.80%	24.80%	27.10%	44.20%
Final conditional default rate trended down to (as a percent)	9.20%	9.20%	8.10%	8.10%
Initial conditional prepayment rate (as a percent)	14.70%	14.70%	9.70%	3.60%
HELOCs
Key Variables
Expected period until final conditional default rate (in months)	36 months	36 months	24 months	21 months
Final conditional prepayment rate (as a percent)	10.00%	10.00%	10.00%	10.00%
Loss severity (as a percent)	98.00%	98.00%	98.00%	95.00%
Period from initial to final draw rate (in months)		3 months
Final draw rate (as a percent)	7.70%	7.70%
Increase in expected loss in case of increase in conditional default rate plateau period		56.9
Decrease in expected loss in case of decrease in conditional default rate ramp down period		53.6
HELOCs | Low end of range
Key Variables
Plateau conditional default rate (as a percent)	4.00%	4.00%	4.20%	10.70%
Final conditional default rate trended down to (as a percent)	0.40%	0.40%	0.40%	0.50%
Initial conditional prepayment rate (as a percent)	1.40%	1.40%	3.30%	1.90%
Initial draw rate (as a percent)	0.00%	0.00%	0.00%	0.10%
Final draw rate in all but one instance (as a percent)	0.00%	0.00%
HELOCs | High end of range
Key Variables
Plateau conditional default rate (as a percent)	27.40%	27.40%	22.10%	40.00%
Final conditional default rate trended down to (as a percent)	3.20%	3.20%	3.20%	3.20%
Initial conditional prepayment rate (as a percent)	25.80%	25.80%	17.50%	14.90%
Initial draw rate (as a percent)	15.30%	15.30%	6.80%	2.00%
Final draw rate in all but one instance (as a percent)	1.30%	1.30%