Investments in Securities Other-than-temporary Impairments 4 (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	$ 12,352
Weighted average collateral default	63.60%	[1]
Weighted average collateral severities	71.70%	[2]
Weighted average voluntary prepayment rates	3.30%	[3]
Average credit enhancement	17.80%	[4]
Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,522
Weighted average collateral default	49.10%	[1]
Weighted average collateral severities	62.80%	[2]
Weighted average voluntary prepayment rates	5.10%	[3]
Average credit enhancement	11.90%	[4]
Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	4,130
Weighted average collateral default	20.40%	[1]
Weighted average collateral severities	55.30%	[2]
Weighted average voluntary prepayment rates	10.30%	[3]
Average credit enhancement	8.10%	[4]
Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,225
Weighted average collateral default	41.10%	[1]
Weighted average collateral severities	58.20%	[2]
Weighted average voluntary prepayment rates	5.90%	[3]
Average credit enhancement	7.10%	[4]
Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	5,363
Weighted average collateral default	22.90%	[1]
Weighted average collateral severities	49.70%	[2]
Weighted average voluntary prepayment rates	8.20%	[3]
Average credit enhancement	5.00%	[4]
2004 and Prior [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,348
Weighted average collateral default	38.20%	[1]
Weighted average collateral severities	66.90%	[2]
Weighted average voluntary prepayment rates	7.70%	[3]
Average credit enhancement	52.10%	[4]
2004 and Prior [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	428
Weighted average collateral default	33.70%	[1]
Weighted average collateral severities	58.50%	[2]
Weighted average voluntary prepayment rates	6.50%	[3]
Average credit enhancement	10.00%	[4]
2004 and Prior [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,702
Weighted average collateral default	12.70%	[1]
Weighted average collateral severities	51.80%	[2]
Weighted average voluntary prepayment rates	12.00%	[3]
Average credit enhancement	12.10%	[4]
2004 and Prior [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	418
Weighted average collateral default	23.40%	[1]
Weighted average collateral severities	51.20%	[2]
Weighted average voluntary prepayment rates	7.90%	[3]
Average credit enhancement	23.30%	[4]
2004 and Prior [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,886
Weighted average collateral default	14.40%	[1]
Weighted average collateral severities	45.10%	[2]
Weighted average voluntary prepayment rates	10.00%	[3]
Average credit enhancement	9.00%	[4]
2005 [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	126
Weighted average collateral default	65.20%	[1]
Weighted average collateral severities	70.70%	[2]
Weighted average voluntary prepayment rates	3.10%	[3]
Average credit enhancement	65.40%	[4]
2005 [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,139
Weighted average collateral default	46.10%	[1]
Weighted average collateral severities	63.70%	[2]
Weighted average voluntary prepayment rates	5.30%	[3]
Average credit enhancement	15.70%	[4]
2005 [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	978
Weighted average collateral default	34.40%	[1]
Weighted average collateral severities	61.10%	[2]
Weighted average voluntary prepayment rates	7.50%	[3]
Average credit enhancement	0.80%	[4]
2005 [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	453
Weighted average collateral default	41.40%	[1]
Weighted average collateral severities	59.70%	[2]
Weighted average voluntary prepayment rates	6.50%	[3]
Average credit enhancement	11.60%	[4]
2005 [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,993
Weighted average collateral default	28.70%	[1]
Weighted average collateral severities	52.10%	[2]
Weighted average voluntary prepayment rates	7.20%	[3]
Average credit enhancement	3.80%	[4]
2006 [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	10,313
Weighted average collateral default	67.00%	[1]
Weighted average collateral severities	72.90%	[2]
Weighted average voluntary prepayment rates	2.70%	[3]
Average credit enhancement	12.30%	[4]
2006 [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	955
Weighted average collateral default	59.50%	[1]
Weighted average collateral severities	63.70%	[2]
Weighted average voluntary prepayment rates	4.20%	[3]
Average credit enhancement	8.30%	[4]
2006 [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	450
Weighted average collateral default	36.80%	[1]
Weighted average collateral severities	63.60%	[2]
Weighted average voluntary prepayment rates	6.00%	[3]
Average credit enhancement	0.10%	[4]
2006 [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,354
Weighted average collateral default	46.50%	[1]
Weighted average collateral severities	59.80%	[2]
Weighted average voluntary prepayment rates	5.10%	[3]
Average credit enhancement	0.60%	[4]
2006 [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,386
Weighted average collateral default	25.20%	[1]
Weighted average collateral severities	52.30%	[2]
Weighted average voluntary prepayment rates	7.20%	[3]
Average credit enhancement	0.00%	[4]
2007 and After [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	565
Weighted average collateral default	62.30%	[1]
Weighted average collateral severities	61.50%	[2]
Weighted average voluntary prepayment rates	2.20%	[3]
Average credit enhancement	26.50%	[4]
2007 and After [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	0
2007 and After [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	0
2007 and After [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	0
2007 and After [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	$ 98
Weighted average collateral default	33.80%	[1]
Weighted average collateral severities	53.10%	[2]
Weighted average voluntary prepayment rates	7.30%	[3]
Average credit enhancement	20.90%	[4]

[1] The expected remaining cumulative default rate of the collateral pool backing the securities, as a percentage of the current collateral unpaid principal balance, weighted by security unpaid principal balance.

[2] The expected remaining loss given default of the collateral pool backing the securities, calculated as the ratio of remaining cumulative loss divided by cumulative defaults, weighted by security unpaid principal balance.

[3] The average monthly voluntary prepayment rate, weighted by security unpaid principal balance.

[4] The average percent current credit enhancement provided by subordination of other securities. Excludes excess interest projections and monoline bond insurance.