Derivatives and Hedging Activities - Effect of Fair Value and Cash Flow Hedges on Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense — Long-term debt		$ (108)	$ (86)	$ (302)	$ (228)
Interest income — Loans		1,025	928	2,965	2,753
Deposits		(140)	(72)	(343)	(196)
Other income	[1]	23	21	143	138
Other Income [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Gains (losses) (before tax) recognized in income for hedge ineffectiveness			0		0
Interest Rate [Member] | Interest Expense, Long-Term Debt [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		41	0	134	0
Recognized on derivatives designated as hedging instruments		(46)	10	(140)	42
Net income (expense) recognized on fair value hedges		(5)	10	(6)	42
Interest Rate [Member] | Interest Expense, Long-Term Debt [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		(2)	(1)	(2)	(3)
Gains (losses) (before tax) recognized in income for hedge ineffectiveness			0		0
Net income (expense) recognized on cash flow hedges		(2)	(1)	(2)	(3)
Interest Rate [Member] | Interest Income [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		0	0	0	0
Recognized on derivatives designated as hedging instruments		0	0	0	0
Net income (expense) recognized on fair value hedges		0	0	0	0
Interest Rate [Member] | Interest Income [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		(15)	1	(42)	21
Gains (losses) (before tax) recognized in income for hedge ineffectiveness			0
Net income (expense) recognized on cash flow hedges		(15)	1	(42)	21
Interest Rate [Member] | Interest Expense, Deposits [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		0	0	1	0
Recognized on derivatives designated as hedging instruments		0	0	0	0
Net income (expense) recognized on fair value hedges		0	0	1	0
Interest Rate [Member] | Interest Expense, Deposits [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		0	0	0	0
Gains (losses) (before tax) recognized in income for hedge ineffectiveness			0		0
Net income (expense) recognized on cash flow hedges		0	0	0	0
Interest Rate [Member] | Other Income [Member] | Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedges Recognized in Earnings [Abstract]
Recognized on hedged items		0	20	0	50
Recognized on derivatives designated as hedging instruments		0	(19)	0	(49)
Net income (expense) recognized on fair value hedges		0	1	0	1
Interest Rate [Member] | Other Income [Member] | Cash Flow Hedging [Member]
Cash Flow Hedges Derivative Instruments at Fair Value, Net [Abstract]
Realized gains (losses) (pre-tax) reclassified from AOCI into net income		0	0	0	0
Net income (expense) recognized on cash flow hedges		$ 0	$ 0	$ 0	$ 0

[1] For the three and nine months ended September 30, 2018, net securities gains totaled less than $1 million. For the three and nine months ended September 30, 2017, net securities gains totaled $1 million. For the three and nine months ended September 30, 2018, and September 30, 2017, we did not have any impairment losses related to securities.