Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Demand Notes	$ 2,666	$ 3,171
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	3,475	7,350
Securities Sold under Agreements to Repurchase, Gross	967	892
Short-term Debt	7,108	11,413
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements	4	10
Non-derivative Cash Collateral received from counterparties associated with Repurchase Agreements	1	1
Non-derivative Noncash Collateral received from counterparties associated with Repurchase Agreements	1
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	2,666	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	2,666	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	3,475	7,350
Securities Sold under Agreements to Repurchase, Gross	967	892
Short-term Debt	4,442	$ 8,242
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	530
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	437
US Treasury and Government [Member] | 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	276
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | 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	$ 691