Derivative Financial Instruments - Amounts Related to Cash Flow Hedges (Details) - Cash Flow Hedges - Interest Rate Contracts - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	$ 69	$ 10	$ (24)
Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	(12)	$ (15)	(11)
Deposits
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	15
Deposits | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	(1)
Short-term borrowings
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	(3)
Short-term borrowings | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	1
Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) recognized in OCI	57
Long-term debt | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax gain (loss) reclassified from AOCI into interest expense	$ (12)