Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Demand notes	$ 2,575	$ 3,171
Federal Home Loan Bank, advances	3,525	7,350
Financial instruments sold under agreements to repurchase	1,238	892
Total short-term borrowings	7,338	11,413
Non-derivative cash collateral placed with counterparties associated with repurchase agreements	15	10
Non-derivative cash collateral received from counterparties associated with repurchase agreements	0	1
Unsecured debt
Short-term Debt [Line Items]
Demand notes	2,575	3,171
Federal Home Loan Bank, advances	0	0
Financial instruments sold under agreements to repurchase	0	0
Total short-term borrowings	2,575	3,171
Secured debt
Short-term Debt [Line Items]
Demand notes	0	0
Federal Home Loan Bank, advances	3,525	7,350
Financial instruments sold under agreements to repurchase	1,238	892
Total short-term borrowings	4,763	$ 8,242
Maturity within 30 days | Secured debt
Short-term Debt [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	1,096
Maturity within 61 to 90 days | Secured debt
Short-term Debt [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	142
US Treasury | Secured debt
Short-term Debt [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	812
Agency mortgage-backed residential | Secured debt
Short-term Debt [Line Items]
Assets Sold under Agreements to Repurchase, Carrying Amount	$ 426