Derivative Financial Instruments - Amounts Related to Cash Flow Hedges (Details) - Cash Flow Hedges - Interest Rate Contracts - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	$ 26	$ 1	$ 150	$ (42)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	0	$ (13)	(14)	$ 1
Deposits
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	6		35
Deposits | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	1		(2)
Short-term borrowings
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	2		4
Short-term borrowings | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	0		0
Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	18		111
Long-term debt | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	$ (1)		$ (12)