Expected Losses to be Paid (Key Assumptions in Base Case Expected Loss First Lien RMBS) (Details 6) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended	12 Months Ended													3 Months Ended							3 Months Ended		12 Months Ended																																					3 Months Ended		3 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 Total first lien scenario	Dec. 31, 2010 Total first lien	Dec. 31, 2012 Alt-A first lien		Dec. 31, 2011 Alt-A first lien		Dec. 31, 2010 Alt-A first lien		Dec. 31, 2012 First mortgage, option adjustable rate mortgage		Dec. 31, 2011 First mortgage, option adjustable rate mortgage		Dec. 31, 2010 First mortgage, option adjustable rate mortgage		Dec. 31, 2012 First mortgage, subprime		Dec. 31, 2011 First mortgage, subprime		Dec. 31, 2010 First mortgage, subprime		Dec. 31, 2012 RMBS	Dec. 31, 2012 Prime first lien scenario	Dec. 31, 2011 Prime first lien scenario	Dec. 31, 2010 Prime first lien scenario	Dec. 31, 2012 Minimum Alt-A first lien		Dec. 31, 2011 Minimum Alt-A first lien		Dec. 31, 2010 Minimum Alt-A first lien		Dec. 31, 2012 Minimum First mortgage, option adjustable rate mortgage		Dec. 31, 2011 Minimum First mortgage, option adjustable rate mortgage		Dec. 31, 2010 Minimum First mortgage, option adjustable rate mortgage		Dec. 31, 2012 Minimum First mortgage, subprime		Dec. 31, 2011 Minimum First mortgage, subprime		Dec. 31, 2010 Minimum First mortgage, subprime		Dec. 31, 2012 Maximum Alt-A first lien		Dec. 31, 2011 Maximum Alt-A first lien		Dec. 31, 2010 Maximum Alt-A first lien		Dec. 31, 2012 Maximum First mortgage, option adjustable rate mortgage		Dec. 31, 2011 Maximum First mortgage, option adjustable rate mortgage		Dec. 31, 2010 Maximum First mortgage, option adjustable rate mortgage		Dec. 31, 2012 Maximum First mortgage, subprime		Dec. 31, 2011 Maximum First mortgage, subprime		Dec. 31, 2010 Maximum First mortgage, subprime		Dec. 31, 2012 Base Scenario Total first lien	Dec. 31, 2012 Somewhat Stressful Environment [Member] Total first lien	Dec. 31, 2012 Somewhat Stressful Environment [Member] Alt-A first lien	Dec. 31, 2012 Somewhat Stressful Environment [Member] First mortgage, option adjustable rate mortgage	Dec. 31, 2012 Somewhat Stressful Environment [Member] First mortgage, subprime	Dec. 31, 2012 Somewhat Stressful Environment [Member] Prime first lien	Dec. 31, 2012 More Stressful Environment [Member] Alt-A first lien	Dec. 31, 2012 More Stressful Environment [Member] First mortgage, option adjustable rate mortgage	Dec. 31, 2012 More Stressful Environment [Member] First mortgage, subprime	Dec. 31, 2012 More Stressful Environment [Member] Prime first lien	Mar. 31, 2012 More Stressful Environment [Member] Prime first lien	Dec. 31, 2012 Somewhat Less Stressful Environment [Member] Total first lien	Dec. 31, 2012 Somewhat Less Stressful Environment [Member] Alt-A first lien	Dec. 31, 2012 Somewhat Less Stressful Environment [Member] First mortgage, option adjustable rate mortgage	Dec. 31, 2012 Somewhat Less Stressful Environment [Member] First mortgage, subprime	Dec. 31, 2012 Somewhat Less Stressful Environment [Member] Prime first lien	Dec. 31, 2012 Least Stressful Environment [Member] Alt-A first lien	Dec. 31, 2012 Least Stressful Environment [Member] First mortgage, option adjustable rate mortgage	Dec. 31, 2012 Least Stressful Environment [Member] First mortgage, subprime	Dec. 31, 2012 Least Stressful Environment [Member] Prime first lien	Mar. 31, 2012 Least Stressful Environment [Member] Prime first lien
Guarantor Obligations [Line Items]
Period from initial to final conditional prepayment rate (in months)		12 months																																																																							12 months
Initial subprime loss severity rate assumed to be recovered over two years (as a percent)																																																																									40.00%
Period for constant conditional default rate (in months)		24 months
Plateau conditional default rate (as a percent)																										3.80%	[1]	2.80%	[1]	2.40%	[1]	7.00%	[1]	9.60%	[1]	9.80%	[1]	7.30%	[1]	8.30%	[1]	9.00%	[1]	23.20%	[1]	41.30%	[1]	42.10%	[1]	26.10%	[1]	31.50%	[1]	32.70%	[1]	26.20%	[1]	29.90%	[1]	34.60%	[1]
Intermediate Conditional Default Rate																						5.00%				0.80%	[1]	0.60%	[1]	0.40%	[1]	1.40%	[1]	1.90%	[1]	1.50%	[1]	1.50%	[1]	1.70%	[1]	1.30%	[1]	4.60%	[1]	8.30%	[1]	6.30%	[1]	5.20%	[1]	6.30%	[1]	4.90%	[1]	5.20%	[1]	6.00%	[1]	5.20%	[1]
Final conditional default rate trended down to (as a percent)																										0.20%	[1]	0.10%	[1]	0.10%	[1]	0.40%	[1]	0.50%	[1]	0.50%	[1]	0.40%	[1]	0.40%	[1]	0.40%	[1]	1.20%	[1]	2.10%	[1]	2.10%	[1]	1.30%	[1]	1.60%	[1]	1.60%	[1]	1.30%	[1]	1.50%	[1]	1.70%	[1]
Loss severity (as a percent)				65.00%	[1]	65.00%	[1]	60.00%	[1]	65.00%	[1]	65.00%	[1]	60.00%	[1]	90.00%	[1]	90.00%	[1]	80.00%	[1]
Initial conditional prepayment rate (as a percent)																										0.00%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]	0.00%	[1]	39.40%	[1]	37.50%	[1]	37.20%	[1]	10.70%	[1]	29.10%	[1]	18.70%	[1]	17.60%	[1]	16.30%	[1]	17.00%	[1]
Final conditional prepayment rate (as a percent)				15.00%	[1]	15.00%	[1]	15.00%	[1]	15.00%	[1]	15.00%	[1]	15.00%	[1]	15.00%	[1]	15.00%	[1]	15.00%	[1]
Period for which estimated defaults are attributed to loans currently delinquent or in foreclosure		24 months																																																												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			15.00%																																																											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%
Estimated Loss Severity Rate, One through Six Months After Period End		90.00%
Period until which loss severity rate would continue (in months)																6 months
Estimated Loss Severity Rate, Six Through Twelve Months After Period End		80.00%
Period for decline in loss severity (in years)																																																														2 years
Loss severity (as a percent)																																																														40.00%
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%
Number of scenarios weighted in estimating expected losses																							5	5	4
Sensitivity Analysis, Number of Months Recovery Delayed Under Most Stressful Scenario	3 months
Sensitivity Analysis, Number of Months Recovery Expedited Under Least Stressful Scenario	3 months
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						2 years
Increased final loss severity for subprime transactions used to calculate potential change in loss estimate (as a percent)																																																															60.00%
Increase in expected loss in case of increase in conditional default rate plateau period and loss severity recovery period																																																																$ 83	$ 21	$ 121	$ 4
Loss severity recovery period used to calculate potential change in loss estimate in even more stressful scenario (in years)		8 years
Period of decrease in conditional default rate until final rate is achieved (in months)																																																																							15 months	12 months									9 months	12 months
Increase in expected loss in case of increase in loss severity recovery period in an even more stressful scenario																																																																				223	60	188	17
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%
Decrease in expected loss in case of less gradual conditional default rate recovery and changes in initial subprime loss severity rate																																																																										11	27	34	1
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																																																																														$ 82	$ 61	$ 75	$ 1

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