Investments in Securities Other-than-temporary Impairments 4 (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	$ 12,653
Weighted average collateral default	65.80%	[1]
Weighted average collateral severities	75.20%	[2]
Weighted average voluntary prepayment rates	2.80%	[3]
Average credit enhancement	18.60%	[4]
Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,604
Weighted average collateral default	54.10%	[1]
Weighted average collateral severities	65.60%	[2]
Weighted average voluntary prepayment rates	4.60%	[3]
Average credit enhancement	13.90%	[4]
Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	4,315
Weighted average collateral default	21.10%	[1]
Weighted average collateral severities	57.60%	[2]
Weighted average voluntary prepayment rates	10.30%	[3]
Average credit enhancement	8.20%	[4]
Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,298
Weighted average collateral default	46.00%	[1]
Weighted average collateral severities	61.30%	[2]
Weighted average voluntary prepayment rates	4.90%	[3]
Average credit enhancement	7.10%	[4]
Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	5,548
Weighted average collateral default	26.80%	[1]
Weighted average collateral severities	52.20%	[2]
Weighted average voluntary prepayment rates	7.20%	[3]
Average credit enhancement	5.10%	[4]
2004 and Prior [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,404
Weighted average collateral default	39.90%	[1]
Weighted average collateral severities	69.00%	[2]
Weighted average voluntary prepayment rates	6.60%	[3]
Average credit enhancement	51.90%	[4]
2004 and Prior [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	438
Weighted average collateral default	37.30%	[1]
Weighted average collateral severities	60.00%	[2]
Weighted average voluntary prepayment rates	6.20%	[3]
Average credit enhancement	11.40%	[4]
2004 and Prior [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,836
Weighted average collateral default	12.80%	[1]
Weighted average collateral severities	53.70%	[2]
Weighted average voluntary prepayment rates	12.10%	[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	431
Weighted average collateral default	26.60%	[1]
Weighted average collateral severities	53.00%	[2]
Weighted average voluntary prepayment rates	6.50%	[3]
Average credit enhancement	23.00%	[4]
2004 and Prior [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,955
Weighted average collateral default	16.30%	[1]
Weighted average collateral severities	47.10%	[2]
Weighted average voluntary prepayment rates	9.40%	[3]
Average credit enhancement	9.20%	[4]
2005 [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	136
Weighted average collateral default	66.50%	[1]
Weighted average collateral severities	74.30%	[2]
Weighted average voluntary prepayment rates	2.60%	[3]
Average credit enhancement	65.30%	[4]
2005 [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,171
Weighted average collateral default	51.00%	[1]
Weighted average collateral severities	66.10%	[2]
Weighted average voluntary prepayment rates	4.80%	[3]
Average credit enhancement	17.10%	[4]
2005 [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,015
Weighted average collateral default	36.60%	[1]
Weighted average collateral severities	64.50%	[2]
Weighted average voluntary prepayment rates	7.30%	[3]
Average credit enhancement	0.80%	[4]
2005 [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	467
Weighted average collateral default	46.70%	[1]
Weighted average collateral severities	62.70%	[2]
Weighted average voluntary prepayment rates	5.40%	[3]
Average credit enhancement	11.80%	[4]
2005 [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,058
Weighted average collateral default	33.30%	[1]
Weighted average collateral severities	54.70%	[2]
Weighted average voluntary prepayment rates	6.00%	[3]
Average credit enhancement	3.90%	[4]
2006 [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	10,543
Weighted average collateral default	69.20%	[1]
Weighted average collateral severities	76.40%	[2]
Weighted average voluntary prepayment rates	2.40%	[3]
Average credit enhancement	13.10%	[4]
2006 [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	995
Weighted average collateral default	65.20%	[1]
Weighted average collateral severities	67.40%	[2]
Weighted average voluntary prepayment rates	3.60%	[3]
Average credit enhancement	11.20%	[4]
2006 [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	464
Weighted average collateral default	38.30%	[1]
Weighted average collateral severities	66.40%	[2]
Weighted average voluntary prepayment rates	5.80%	[3]
Average credit enhancement	0.20%	[4]
2006 [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,400
Weighted average collateral default	51.80%	[1]
Weighted average collateral severities	63.40%	[2]
Weighted average voluntary prepayment rates	4.20%	[3]
Average credit enhancement	0.60%	[4]
2006 [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,434
Weighted average collateral default	30.90%	[1]
Weighted average collateral severities	55.30%	[2]
Weighted average voluntary prepayment rates	6.00%	[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	570
Weighted average collateral default	64.60%	[1]
Weighted average collateral severities	66.90%	[2]
Weighted average voluntary prepayment rates	1.90%	[3]
Average credit enhancement	27.40%	[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	$ 101
Weighted average collateral default	37.10%	[1]
Weighted average collateral severities	57.20%	[2]
Weighted average voluntary prepayment rates	6.90%	[3]
Average credit enhancement	21.70%	[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.