Expected Losses to be Paid (Details 6) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013		Dec. 31, 2012		Mar. 31, 2013
Total first lien
Guarantor Obligations [Line Items]
Period from initial to final conditional prepayment rate (in months)	12 months
Period for constant conditional default rate (in months)	24 months
Period for which estimated defaults are attributed to loans currently delinquent or in foreclosure	24 months
Estimated Loss Severity Rate, One through Six Months After Period End	90.00%
Estimated Loss Severity Rate, Six Through Twelve Months After Period End	80.00%
Prior conditional prepayment rate used in alternate scenario for loss estimate (as a percent)	10.00%
Current conditional prepayment rate used in alternate scenario for loss estimate (as a percent)	15.00%
Loss severity recovery period used to calculate potential change in loss estimate in even more stressful scenario (in years)	8 years
Number of scenarios where the recovery was faster than in base case	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
Alt-A first lien
Guarantor Obligations [Line Items]
Loss severity (as a percent)	65.00%	[1]	65.00%	[1]	65.00%	[1]
Final conditional prepayment rate (as a percent)	15.00%	[1]	15.00%	[1]	15.00%	[1]
First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Loss severity (as a percent)	65.00%	[1]	65.00%	[1]	65.00%	[1]
Final conditional prepayment rate (as a percent)	15.00%	[1]	15.00%	[1]	15.00%	[1]
First mortgage, subprime
Guarantor Obligations [Line Items]
Loss severity (as a percent)	90.00%	[1]	90.00%	[1]	90.00%	[1]
Final conditional prepayment rate (as a percent)	15.00%	[1]	15.00%	[1]	15.00%	[1]
Period until which loss severity rate would continue (in months)	6 months		6 months
RMBS
Guarantor Obligations [Line Items]
Intermediate Conditional Default Rate	5.00%
Prime first lien
Guarantor Obligations [Line Items]
Number of scenarios weighted in estimating expected losses	5		5
Minimum | Alt-A first lien
Guarantor Obligations [Line Items]
Plateau conditional default rate (as a percent)	3.70%	[1]	3.80%	[1]	3.70%	[1]
Intermediate Conditional Default Rate	0.70%	[1]	0.80%	[1]	0.70%	[1]
Final conditional default rate trended down to (as a percent)	0.20%	[1]	0.20%	[1]	0.20%	[1]
Initial conditional prepayment rate (as a percent)	0.40%	[1]	0.00%	[1]	0.10%	[1]
Minimum | First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Plateau conditional default rate (as a percent)	5.60%	[1]	7.00%	[1]	6.40%	[1]
Intermediate Conditional Default Rate	1.10%	[1]	1.40%	[1]	1.30%	[1]
Final conditional default rate trended down to (as a percent)	0.30%	[1]	0.40%	[1]	0.30%	[1]
Initial conditional prepayment rate (as a percent)	0.30%	[1]	0.00%	[1]	0.30%	[1]
Minimum | First mortgage, subprime
Guarantor Obligations [Line Items]
Plateau conditional default rate (as a percent)	6.70%	[1]	7.30%	[1]	7.80%	[1]
Intermediate Conditional Default Rate	1.30%	[1]	1.50%	[1]	1.60%	[1]
Final conditional default rate trended down to (as a percent)	0.30%	[1]	0.40%	[1]	0.40%	[1]
Initial conditional prepayment rate (as a percent)	0.00%	[1]	0.00%	[1]	0.00%	[1]
Maximum | Alt-A first lien
Guarantor Obligations [Line Items]
Plateau conditional default rate (as a percent)	22.30%	[1]	23.20%	[1]	22.60%	[1]
Intermediate Conditional Default Rate	4.50%	[1]	4.60%	[1]	4.50%	[1]
Final conditional default rate trended down to (as a percent)	1.10%	[1]	1.20%	[1]	1.10%	[1]
Initial conditional prepayment rate (as a percent)	32.20%	[1]	39.40%	[1]	39.60%	[1]
Maximum | First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Plateau conditional default rate (as a percent)	24.20%	[1]	26.10%	[1]	25.20%	[1]
Intermediate Conditional Default Rate	4.80%	[1]	5.20%	[1]	5.00%	[1]
Final conditional default rate trended down to (as a percent)	1.20%	[1]	1.30%	[1]	1.30%	[1]
Initial conditional prepayment rate (as a percent)	7.70%	[1]	10.70%	[1]	10.60%	[1]
Maximum | First mortgage, subprime
Guarantor Obligations [Line Items]
Plateau conditional default rate (as a percent)	24.70%	[1]	26.20%	[1]	25.60%	[1]
Intermediate Conditional Default Rate	4.90%	[1]	5.20%	[1]	5.10%	[1]
Final conditional default rate trended down to (as a percent)	1.20%	[1]	1.30%	[1]	1.30%	[1]
Initial conditional prepayment rate (as a percent)	14.80%	[1]	17.60%	[1]	14.70%	[1]
Base Scenario | Total first lien
Guarantor Obligations [Line Items]
Period for which estimated defaults are attributed to loans currently delinquent or in foreclosure	24 months
Period from plateau to intermediate conditional default rate (in months)	12 months
Intermediate conditional default rate as a percentage of plateau conditional default rate	20.00%
Period of constant intermediate conditional default rate (in months)	36 months
Final conditional default rate as a percentage of plateau conditional default rate	5.00%
Period for decline in loss severity (in years)	2 years
Loss severity (as a percent)	40.00%
Somewhat Stressful Environment [Member] | Total first lien
Guarantor Obligations [Line Items]
Increased final loss severity for subprime transactions used to calculate potential change in loss estimate (as a percent)	60.00%
Somewhat Stressful Environment [Member] | Alt-A first lien
Guarantor Obligations [Line Items]
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	$ 70
Somewhat Stressful Environment [Member] | First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	20
Somewhat Stressful Environment [Member] | First mortgage, subprime
Guarantor Obligations [Line Items]
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period	105
Somewhat Stressful Environment [Member] | Prime first lien
Guarantor Obligations [Line Items]
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
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	6
More Stressful Environment [Member] | Alt-A first lien
Guarantor Obligations [Line Items]
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	192
More Stressful Environment [Member] | First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	47
More Stressful Environment [Member] | First mortgage, subprime
Guarantor Obligations [Line Items]
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	166
More Stressful Environment [Member] | Prime first lien
Guarantor Obligations [Line Items]
Period of decrease in conditional default rate until final rate is achieved (in months)	15 months
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario	16
Somewhat Less Stressful Environment [Member] | Total first lien
Guarantor Obligations [Line Items]
Period from initial to final conditional prepayment rate (in months)	12 months
Initial subprime loss severity rate assumed to be recovered over two years (as a percent)	40.00%
Current loss severity recovery period used to calculate potential change in loss estimate (in years)	2 years
Initial subprime loss severity rate assumed for 12 months (as a percent)	80.00%
Somewhat Less Stressful Environment [Member] | Alt-A first lien
Guarantor Obligations [Line Items]
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	6
Somewhat Less Stressful Environment [Member] | First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	15
Somewhat Less Stressful Environment [Member] | First mortgage, subprime
Guarantor Obligations [Line Items]
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	33
Somewhat Less Stressful Environment [Member] | Prime first lien
Guarantor Obligations [Line Items]
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate	1
Least Stressful Environment [Member] | Alt-A first lien
Guarantor Obligations [Line Items]
Decrease in expected loss in case of decrease in conditional default rate plateau period	66
Least Stressful Environment [Member] | First mortgage, option adjustable rate mortgage
Guarantor Obligations [Line Items]
Decrease in expected loss in case of decrease in conditional default rate plateau period	42
Least Stressful Environment [Member] | First mortgage, subprime
Guarantor Obligations [Line Items]
Decrease in expected loss in case of decrease in conditional default rate plateau period	76
Least Stressful Environment [Member] | Prime first lien
Guarantor Obligations [Line Items]
Period of decrease in conditional default rate until final rate is achieved (in months)	9 months
Decrease in expected loss in case of decrease in conditional default rate plateau period	$ 6

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