Derivative Financial Instruments - Notional Amount of Interest Rate Swap Agreements Designated as Fair Value and Cash Flow Hedges (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Total	$ 11,047	$ 10,169
Investment securities available-for-sale
Derivative [Line Items]
Investment securities available-for-sale	1,254	1,444
Long-term debt
Derivative [Line Items]
Long-term debt	8,493	8,725
Floating-rate loans
Derivative [Line Items]
Long-term debt	1,300	0
Fair Value Hedges
Derivative [Line Items]
Total	9,747	10,169
Increase in carrying value of long-term debt	87	15
Fair Value Hedges | Investment securities available-for-sale
Derivative [Line Items]
Investment securities available-for-sale	1,254	1,444
Fair Value Hedges | Long-term debt
Derivative [Line Items]
Long-term debt	8,493	8,725
Fair Value Hedges | Floating-rate loans
Derivative [Line Items]
Long-term debt	0	0
Cash Flow Hedges
Derivative [Line Items]
Total	1,300	0
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	$ 0