Derivatives and Hedging Activities - Effect of Fair Value and Cash Flow Hedges on Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense — Long-term debt		$ (102)	$ (74)	$ (194)	$ (142)
Interest income — Loans		1,000	948	1,940	1,825
Deposits		(112)	(66)	(203)	(124)
Other income	[1]	99	85	120	117
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		22	0	93	0
Recognized on derivatives designated as hedging instruments		(25)	14	(94)	32
Net income (expense) recognized on fair value hedges		(3)	14	(1)	32
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		1	(1)	0	(2)
Gains (losses) (before tax) recognized in income for hedge ineffectiveness			0		0
Net income (expense) recognized on cash flow hedges		1	(1)	0	(2)
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		(25)	5	(27)	20
Gains (losses) (before tax) recognized in income for hedge ineffectiveness			0
Net income (expense) recognized on cash flow hedges		(25)	5	(27)	20
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		1	0	1	0
Recognized on derivatives designated as hedging instruments		0	0	0	0
Net income (expense) recognized on fair value hedges		1	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	(5)	0	30
Recognized on derivatives designated as hedging instruments		0	5	0	(30)
Net income (expense) recognized on fair value hedges		0	0	0	0
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 months ended June 30, 2018, net securities gains totaled less than $1 million. For the three months ended June 30, 2017, net securities gains totaled $1 million. For the three months ended June 30, 2018, and June 30, 2017, we did not have any impairment losses related to securities.