Fair Value - Fair Value Inputs, Assets and Liabilities, Quantitative Information (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	$ 44,304	$ 57,121
Asset-backed securities, other
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	826	1,447
State and political subdivisions
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	$ 7,307	$ 9,151
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Option model | Derivative instruments, liabilities | Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Volatility	0.00%	7.20%
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Option model | Derivative instruments, assets | Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Volatility	0.00%	7.20%
Significant Unobservable Inputs Readily Available | Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Total	$ 3	$ 1
Total	2	1
Significant Unobservable Inputs Readily Available | Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Derivative instruments, liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative instruments, foreign exchange contracts	2	1
Significant Unobservable Inputs Readily Available | Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Derivative instruments, assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative instruments, foreign exchange contracts	$ 3	$ 1