Investments in Securities Other-than-temporary Impairments 4 (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	$ 12,960
Weighted average collateral default	67.00%	[1]
Weighted average collateral severities	76.40%	[2]
Weighted average voluntary prepayment rates	2.70%	[3]
Average credit enhancement	19.60%	[4]
Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,689
Weighted average collateral default	56.10%	[1]
Weighted average collateral severities	66.80%	[2]
Weighted average voluntary prepayment rates	4.60%	[3]
Average credit enhancement	16.30%	[4]
Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	4,507
Weighted average collateral default	21.80%	[1]
Weighted average collateral severities	58.30%	[2]
Weighted average voluntary prepayment rates	10.50%	[3]
Average credit enhancement	8.20%	[4]
Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,373
Weighted average collateral default	49.60%	[1]
Weighted average collateral severities	62.00%	[2]
Weighted average voluntary prepayment rates	5.10%	[3]
Average credit enhancement	7.10%	[4]
Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	5,751
Weighted average collateral default	29.20%	[1]
Weighted average collateral severities	52.90%	[2]
Weighted average voluntary prepayment rates	7.60%	[3]
Average credit enhancement	5.30%	[4]
2004 and Prior [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,459
Weighted average collateral default	40.50%	[1]
Weighted average collateral severities	69.80%	[2]
Weighted average voluntary prepayment rates	6.40%	[3]
Average credit enhancement	51.80%	[4]
2004 and Prior [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	448
Weighted average collateral default	38.40%	[1]
Weighted average collateral severities	60.60%	[2]
Weighted average voluntary prepayment rates	6.30%	[3]
Average credit enhancement	12.20%	[4]
2004 and Prior [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,973
Weighted average collateral default	13.10%	[1]
Weighted average collateral severities	54.30%	[2]
Weighted average voluntary prepayment rates	12.40%	[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	445
Weighted average collateral default	28.70%	[1]
Weighted average collateral severities	53.40%	[2]
Weighted average voluntary prepayment rates	7.00%	[3]
Average credit enhancement	22.70%	[4]
2004 and Prior [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,027
Weighted average collateral default	17.10%	[1]
Weighted average collateral severities	47.40%	[2]
Weighted average voluntary prepayment rates	9.90%	[3]
Average credit enhancement	9.40%	[4]
2005 [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	145
Weighted average collateral default	67.80%	[1]
Weighted average collateral severities	75.60%	[2]
Weighted average voluntary prepayment rates	2.40%	[3]
Average credit enhancement	64.90%	[4]
2005 [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,203
Weighted average collateral default	53.00%	[1]
Weighted average collateral severities	67.50%	[2]
Weighted average voluntary prepayment rates	4.90%	[3]
Average credit enhancement	20.70%	[4]
2005 [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,056
Weighted average collateral default	38.50%	[1]
Weighted average collateral severities	65.50%	[2]
Weighted average voluntary prepayment rates	7.30%	[3]
Average credit enhancement	0.90%	[4]
2005 [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	484
Weighted average collateral default	50.20%	[1]
Weighted average collateral severities	64.00%	[2]
Weighted average voluntary prepayment rates	5.50%	[3]
Average credit enhancement	12.40%	[4]
2005 [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	2,124
Weighted average collateral default	36.80%	[1]
Weighted average collateral severities	55.10%	[2]
Weighted average voluntary prepayment rates	6.40%	[3]
Average credit enhancement	4.10%	[4]
2006 [Member] | Subprime private-label securities [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	10,781
Weighted average collateral default	70.60%	[1]
Weighted average collateral severities	77.70%	[2]
Weighted average voluntary prepayment rates	2.30%	[3]
Average credit enhancement	14.10%	[4]
2006 [Member] | Alt-A Option ARM [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,038
Weighted average collateral default	67.30%	[1]
Weighted average collateral severities	68.60%	[2]
Weighted average voluntary prepayment rates	3.60%	[3]
Average credit enhancement	13.00%	[4]
2006 [Member] | Alt-A Fixed Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	478
Weighted average collateral default	39.50%	[1]
Weighted average collateral severities	67.40%	[2]
Weighted average voluntary prepayment rates	5.80%	[3]
Average credit enhancement	0.40%	[4]
2006 [Member] | Alt-A Variable Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,444
Weighted average collateral default	55.80%	[1]
Weighted average collateral severities	64.00%	[2]
Weighted average voluntary prepayment rates	4.30%	[3]
Average credit enhancement	0.60%	[4]
2006 [Member] | Alt-A Hybrid Rate [Member]
UPB and Modeled Attributes [Line Items]
Unpaid principal balance	1,495
Weighted average collateral default	34.10%	[1]
Weighted average collateral severities	56.80%	[2]
Weighted average voluntary prepayment rates	6.30%	[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	575
Weighted average collateral default	66.30%	[1]
Weighted average collateral severities	69.80%	[2]
Weighted average voluntary prepayment rates	2.00%	[3]
Average credit enhancement	28.80%	[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	$ 105
Weighted average collateral default	40.10%	[1]
Weighted average collateral severities	59.00%	[2]
Weighted average voluntary prepayment rates	6.90%	[3]
Average credit enhancement	23.20%	[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.