Pledged Assets (Assets Pledged as Collateral) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Offsetting Assets and Liabilities [Line Items]
Retained Interests in Consolidated VIE's Pledged as Collateral Under Repurchase Agreements	$ 194	$ 245
Federal Home Loan Bank Stock	126	150
Available-for-sale Securities Pledged as Collateral	44,089	48,380
Accrued interest on pledged securities	125	140
Restricted cash and cash equivalents	681	1,281
Total Fair Value of Securities Pledged and Accrued Interest	46,354
Total Fair Value Of Agency Securities Pledged And Accrued Interest		51,213
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	42,997	47,992
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	684	148
Accrued interest on pledged securities		0
Restricted cash and cash equivalents	666	1,226
Total Fair Value of Securities Pledged and Accrued Interest	1,352
Total Fair Value Of Agency Securities Pledged And Accrued Interest		1,374
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Accrued interest on pledged securities	118	135
Total Fair Value of Securities Pledged and Accrued Interest	43,277
Total Fair Value Of Agency Securities Pledged And Accrued Interest		48,288
Variable Interest Entity, Primary Beneficiary [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	890	1,029
Accrued interest on pledged securities	3	3
Total Fair Value of Securities Pledged and Accrued Interest	893
Total Fair Value Of Agency Securities Pledged And Accrued Interest		1,032
Under Prime Broker Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	830	485
Accrued interest on pledged securities	2	2
Total Fair Value of Securities Pledged and Accrued Interest	832
Total Fair Value Of Agency Securities Pledged And Accrued Interest		519
Includes Sold But Not Yet Settled Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	45,401	49,654
AAA Non-Agency Mortgage-Backed Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	102	113
AAA Non-Agency Mortgage-Backed Securities [Member] | Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	102	113
US Treasury Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	45	25
US Treasury Securities [Member] | Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	45	25
US Treasury Securities [Member] | Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Available-for-sale Securities Pledged as Collateral	0	0
Derivative [Member]
Offsetting Assets and Liabilities [Line Items]
Accrued interest on pledged securities	2
Repurchase Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash and cash equivalents	15	23
Under Prime Broker Agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash and cash equivalents	0	32
Excluding Cash Received [Member]
Offsetting Assets and Liabilities [Line Items]
Restricted cash and cash equivalents	$ 681	$ 1,281