Debt Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Demand Notes	$ 3,379	$ 3,622
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	5,625	7,875
Securities Sold under Agreements to Repurchase, Gross	1,171	1,176
Short-term Debt	10,175	12,673
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements	10	45
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	3,379	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
Short-term Debt	3,379	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	5,625	7,875
Securities Sold under Agreements to Repurchase, Gross	1,171	1,176
Short-term Debt	6,796	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	537
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	480
Maturity less than 60 Days [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	634
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	$ 154
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		$ 500