Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Demand Notes	$ 2,957	$ 3,171
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	5,900	7,350
Securities Sold under Agreements to Repurchase, Gross	707	892
Short-term Debt	9,564	11,413
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements	5	10
Non-derivative Cash Collateral received from counterparties associated with Repurchase Agreements	1	1
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	2,957	3,171
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	2,957	3,171
Secured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	0	0
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	5,900	7,350
Securities Sold under Agreements to Repurchase, Gross	707	892
Short-term Debt	6,607	$ 8,242
Maturity less than 90 Days [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	707
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	432
Maturity 61 to 90 Days [Member] | Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	$ 275