Fair Value - Fair Value Inputs, Assets and Liabilities, Quantitative Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	$ 56,238	$ 61,998
Asset-backed securities, other
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	1,221	905
State and political subdivisions
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	$ 9,738	$ 10,322
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Discounted cash flows | Asset-backed securities, other | Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Credit spread	0.00%	0.30%
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Discounted cash flows | State and political subdivisions | Weighted average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Credit spread	1.70%	1.80%
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	8.30%	14.40%
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	8.30%	14.40%
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	$ 40	$ 48
Total	2	8
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	8
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	2	8
Significant Unobservable Inputs Readily Available | Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Asset-backed securities, other
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	0	1
Significant Unobservable Inputs Readily Available | Pricing Methods with Significant Unobservable Market Inputs (Level 3) | State and political subdivisions
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Investment securities available-for-sale	$ 38	$ 39