Pledged Assets (Securities Pledged as Collateral) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Retained Interests in Consolidated VIE's Pledged as Collateral Under Repo	$ 256	$ 179
Security Owned and Pledged as Collateral, Fair Value	47,260	52,941
Agency Securities Pledged As Collateral Amortized Cost	46,739	52,397
Agency Securities Pledged As Collateral Accrued Interest	130	147
Maturity Less than 30 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	18,800	14,659
Agency Securities Pledged As Collateral Amortized Cost	18,548	14,509
Agency Securities Pledged As Collateral Accrued Interest	51	41
Maturity 31 To 59 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	5,626	10,906
Agency Securities Pledged As Collateral Amortized Cost	5,582	10,784
Agency Securities Pledged As Collateral Accrued Interest	16	30
Maturity 60 To 90 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	4,820	10,205
Agency Securities Pledged As Collateral Amortized Cost	4,774	10,109
Agency Securities Pledged As Collateral Accrued Interest	13	28
Maturity over 90 days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	17,552	15,267
Agency Securities Pledged As Collateral Amortized Cost	17,375	15,096
Agency Securities Pledged As Collateral Accrued Interest	49	43
Maturity Overnight [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	209	1,904
Agency Securities Pledged As Collateral Amortized Cost	207	1,899
Agency Securities Pledged As Collateral Accrued Interest	0	5
2 Days to 1 Month [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	253	0
Agency Securities Pledged As Collateral Amortized Cost	253	0
Agency Securities Pledged As Collateral Accrued Interest	1	0
Agency Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	46,798	51,037
Agency Securities Pledged As Collateral Amortized Cost	46,279	50,498
Agency Securities Pledged As Collateral Accrued Interest	129	142
US Treasury Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	462	1,904
Agency Securities Pledged As Collateral Amortized Cost	460	1,899
Agency Securities Pledged As Collateral Accrued Interest	$ 1	$ 5