Derivative Financial Instruments - Notional Amount of Interest Rate Swap Agreements Designated as Fair Value and Cash Flow Hedges (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Total	$ 10,994	$ 11,047
Investment securities available-for-sale
Derivative [Line Items]
Investment securities available-for-sale	1,201	1,254
Long-term debt
Derivative [Line Items]
Long-term debt	8,493	8,493
Floating-rate loans
Derivative [Line Items]
Long-term debt	1,300	1,300
Fair Value Hedges
Derivative [Line Items]
Total	9,694	9,747
Increase in carrying value of long-term debt	(347)	(87)
Fair Value Hedges | Investment securities available-for-sale
Derivative [Line Items]
Investment securities available-for-sale	1,201	1,254
Fair Value Hedges | Long-term debt
Derivative [Line Items]
Long-term debt	8,493	8,493
Fair Value Hedges | Floating-rate loans
Derivative [Line Items]
Long-term debt	0	0
Cash Flow Hedges
Derivative [Line Items]
Total	1,300	1,300
Cash Flow Hedges | Investment securities available-for-sale
Derivative [Line Items]
Investment securities available-for-sale	0	0
Cash Flow Hedges | Long-term debt
Derivative [Line Items]
Long-term debt	0	0
Cash Flow Hedges | Floating-rate loans
Derivative [Line Items]
Long-term debt	$ 1,300	$ 1,300