Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Demand notes	$ 2,501	$ 2,477
Federal Home Loan Bank, advances	2,375	6,825
Securities sold under agreements to repurchase	459	685
Total short-term borrowings	5,335	9,987
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements		0
Non-derivative cash collateral received from counterparties associated with repurchase agreements	6	8
Non-derivative noncash collateral received from counterparties associated with repurchase agreements		4
Unsecured debt
Short-term Debt [Line Items]
Demand notes	2,501	2,477
Federal Home Loan Bank, advances	0	0
Securities sold under agreements to repurchase	0	0
Total short-term borrowings	2,501	2,477
Secured debt
Short-term Debt [Line Items]
Demand notes	0	0
Federal Home Loan Bank, advances	2,375	6,825
Securities sold under agreements to repurchase	459	685
Total short-term borrowings	2,834	$ 7,510
Secured debt | Maturity within 30 days
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	145
Secured debt | Maturity within 31 to 60 days
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	170
Secured debt | Maturity within 61 to 90 Days
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	144
Secured debt | Agency mortgage-backed residential
Short-term Debt [Line Items]
Financial instruments sold under agreements to repurchase	$ 459