Derivative Financial Instruments - Amounts Related to Cash Flow Hedges (Details) - Cash Flow Hedges - Interest Rate Contracts - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	$ 31	$ (47)	$ 124	$ (43)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	(3)	$ 6	(14)	$ 14
Deposits
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	8		29
Deposits | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	(1)		(3)
Short-term borrowings
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	2		2
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	21		93
Long-term debt | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	$ (2)		$ (11)