Expected Losses to be Paid (Key Assumptions in Base Case Expected Loss Second Lien RMBS) (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended	3 Months Ended		12 Months Ended	3 Months Ended														3 Months Ended												3 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended		1 Months Ended	3 Months Ended		12 Months Ended	3 Months Ended
	Mar. 31, 2013 contract	Dec. 31, 2012 Payment	Mar. 31, 2013 Second lien Curve	Dec. 31, 2012 Second lien	Dec. 31, 2011 Second lien	Mar. 31, 2013 Second lien Minimum	Mar. 31, 2013 Second lien Maximum	Mar. 31, 2013 Close ended second lien		Dec. 31, 2012 Close ended second lien		Mar. 31, 2013 Close ended second lien Minimum		Dec. 31, 2012 Close ended second lien Minimum		Mar. 31, 2013 Close ended second lien Maximum		Dec. 31, 2012 Close ended second lien Maximum		Mar. 31, 2013 HELOCs		Dec. 31, 2012 HELOCs		Mar. 31, 2013 HELOCs Minimum		Dec. 31, 2012 HELOCs Minimum		Mar. 31, 2013 HELOCs Maximum		Dec. 31, 2012 HELOCs Maximum		Mar. 31, 2013 Base Scenario Second lien	Dec. 31, 2011 Base Scenario Second lien	Jan. 31, 2012 Scenario 1 [Member] Second lien	Mar. 31, 2013 Scenario 1 [Member] Second lien	Dec. 31, 2011 Scenario 1 [Member] Second lien	Mar. 31, 2013 Scenario 1 [Member] Close ended second lien	Mar. 31, 2013 Scenario 1 [Member] HELOCs	Jan. 31, 2012 Scenario 2 [Member] Second lien	Mar. 31, 2013 Scenario 2 [Member] Second lien	Dec. 31, 2012 Scenario 2 [Member] Second lien	Dec. 31, 2011 Scenario 2 [Member] Second lien	Mar. 31, 2013 Scenario 2 [Member] Close ended second lien	Mar. 31, 2013 Scenario 2 [Member] HELOCs
Guarantor Obligations [Line Items]
Typical past due period for loans to be charged off (in days)			180 days
Maximum number of payments behind to be considered performing borrower		1
Plateau conditional default rate (as a percent)												6.70%	[1]	7.30%	[1]	18.60%	[1]	20.70%	[1]					5.00%	[1]	3.80%	[1]	17.20%	[1]	15.90%	[1]
Final conditional default rate trended down to (as a percent)												3.50%	[1]	3.50%	[1]	9.10%	[1]	9.10%	[1]					0.40%	[1]	0.40%	[1]	3.20%	[1]	3.20%	[1]
Conditional Default Rate Stress Period								36 months	[1]	36 months										36 months		36 months	[1]
Initial conditional prepayment rate (as a percent)												2.70%	[1]	1.90%	[1]	13.40%	[1]	12.50%	[1]					2.40%	[1]	2.90%	[1]	18.90%	[1]	15.40%	[1]
Final conditional prepayment rate (as a percent)			10.00%					10.00%	[1]	10.00%	[1]									10.00%	[1]	10.00%	[1]
Loss severity (as a percent)								98.00%	[1]	98.00%	[1]									98.00%	[1]	98.00%	[1]
Initial draw rate (as a percent)																								0.00%	[1]	0.00%	[1]	3.30%	[1]	4.80%	[1]
Period Company Reduced Most Optimistic Case for Market Recovery	3 months
Period Company Extended Most Pessimistic Case for Market Recovery	3 months
Number of Types of Second Lien Residential Mortgage Back Securities Insured	2
First delinquency category (in days)						30 days	59 days
Second delinquency category (in days)						60 days	89 days
Third delinquency category (in days)						90 days	119 days
Fourth delinquency category (in days)						120 days	149 days
Fifth delinquency category (in days)						150 days	179 days
Period of Default Estimate			5 months
Number of Preceding Months Liquidation Rates Used to Estimate Loan Default Rate			12 months
Number of Months the Conditional Default Rate is Calculated Applying Liquidation Rates to Current Period Past Due Balances Excluding Thirty to Fifty Nine Days Past Due			4 months
Period of decrease in conditional default rate until final rate is achieved (in months)			30 months																													30 months	30 months		33 months	30 months						24 months
Stress period (in months)			36 months																													36 months	36 months		42 months	39 months				27		30 months
Number of Monthly Delinquency Categories			5 months
Period of constant conditional default rate (in months)			1 month																													1 month	1 month							1 month
Decreased ramp down period used to calculate potential change in loss estimate (in months)																																								21 months
Loss Recovery Assumption			2.00%
Conditional Prepayment Rate Base Case Average Number of Months			3 months
Period from Initial to Final Draw Rate																				3 months
Final Draw Rate																								0.00%				1.70%
Number of conditional default rate curves modeled in estimating losses			3
LengthOfTotalStressPeriodInPessimisticScenariosInSensitivityAnalysis																																									3 months
Increased plateau period used to calculate potential change in loss estimate (in months)					1 month																													4 months	4 months	4 months			1 month
Loss Estimate Sensitivity Analysis Increase in Conditional Default Rate Ramp Down Period	3 months
Loss Estimate Sensitivity Analysis Change in Estimate for Increased Conditional Default Rate Plateau Period																																					$ 1	$ 44
Loss Estimate Sensitivity Analysis Change in Estimate for Decreased Conditional Default Rate Ramp Down Period																																											$ 4	$ 45
Number of Fewer Months in Optimistic Scenario Compared to Previously Years Total Stress Period				3 months

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