Debt Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Demand Notes	$ 3,457	$ 3,622
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	4,300	7,875
Securities Sold under Agreements to Repurchase, Gross	1,706	1,176
Other Short-term Borrowings	1,249	0
Short-term Debt	10,712	12,673
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements	11	45
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	3,457	3,622
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	0	0
Securities Sold under Agreements to Repurchase, Gross	0	0
Other Short-term Borrowings	1,249	0
Short-term Debt	4,706	3,622
Secured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	0	0
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	4,300	7,875
Securities Sold under Agreements to Repurchase, Gross	1,706	1,176
Other Short-term Borrowings	0	0
Short-term Debt	6,006	9,051
Maturity Less than 30 Days [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
U.S. Treasury Securities Sold under Agreements to Repurchase, Maturity Period	139
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	652
Maturity 31 to 60 Days [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	348
Maturity 61 to 90 Days [Member] [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	183
One year, or less [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
Asset-backed Automotive Financial Securities Sold under Agreements to Repurchase, Maturity Period	384	$ 500
Committed Funding Facilities Member | Nonbank Funding Member | Unsecured Debt [Member]
Short-term Debt [Line Items]
Debt Issuance Costs, Net	$ 1