Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Demand notes	$ 2,477	$ 3,171
Federal Home Loan Bank, advances	6,825	7,350
Financial instruments sold under agreements to repurchase	685	892
Total short-term borrowings	$ 9,987	$ 11,413
Weighted average interest rate	2.50%	1.50%
Non-derivative cash collateral received from counterparties associated with repurchase agreements	$ 8	$ 1
Non-derivative noncash collateral received from counterparties associated with repurchase agreements	4
Non-derivative cash collateral placed with counterparties associated with repurchase agreements	0	10
Unsecured debt
Short-term Debt [Line Items]
Demand notes	2,477	3,171
Federal Home Loan Bank, advances	0	0
Financial instruments sold under agreements to repurchase	0	0
Total short-term borrowings	2,477	3,171
Secured debt
Short-term Debt [Line Items]
Demand notes	0	0
Federal Home Loan Bank, advances	6,825	7,350
Financial instruments sold under agreements to repurchase	685	892
Total short-term borrowings	7,510	$ 8,242
Maturity within 30 days | Secured debt
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	575
Maturity within 31 to 60 days | Secured debt
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	110
US Treasury | Secured debt
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	436
Agency mortgage-backed residential | Secured debt
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	$ 249