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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	$ (79)	$ 31	$ (119)	$ 124
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	(2)	(3)	3	(14)
Deposits
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	(33)	8	(43)	29
Deposits | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	(1)	(1)	1	(3)
Short-term borrowings
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	12	2	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	1	0
Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	(58)	21	(78)	93
Long-term debt | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	$ (1)	$ (2)	$ 1	$ (11)