Derivative Financial Instruments - Amounts Related to Cash Flow Hedges (Details) - Cash Flow Hedges - Interest Rate Contracts - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	$ (40)	$ 93
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	5	(11)
Deposits
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	(10)	21
Deposits | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	2	(2)
Short-term borrowings
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	(10)	0
Short-term borrowings | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	1	0
Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	(20)	72
Long-term debt | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	$ 2	$ (9)