Derivative Financial Instruments - Classifications and Hedging Relationships (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	$ 68,400	$ 75,172
Fair Value, Gain	186	443
Fair Value, Loss	(439)	(714)
Derivative Asset [Abstract]
Amounts subject to master netting arrangements not offset due to policy election	(71)	(297)
Cash collateral (received) posted	(62)	(20)
Net amount	53	126
Derivative Liability [Abstract]
Amounts subject to master netting arrangements not offset due to policy election	71	297
Cash collateral (received) posted	112	344
Net amount	(256)	(73)
Cash flow hedges \| Interest rate contracts \| Pay fixed swaps \| 3 mo. LIBOR funding
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	6,500	6,500
Fair Value, Gain	0	0
Fair Value, Loss	0	(126)
Fair value hedges \| Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	19,773	22,272
Fair Value, Gain	3	123
Fair Value, Loss	(156)	(244)
Fair value hedges \| Interest rate contracts \| Pay fixed swaps \| Commercial loans
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	521	416
Fair Value, Gain	3	5
Fair Value, Loss	0	(1)
Fair value hedges \| Interest rate contracts \| Pay fixed swaps \| Municipal securities
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	259	231
Fair Value, Gain	0	0
Fair Value, Loss	0	(76)
Fair value hedges \| Interest rate contracts \| Receive fixed swaps \| Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	13,558	15,538
Fair Value, Gain	0	118
Fair Value, Loss	(155)	(166)
Fair value hedges \| Interest rate contracts \| Options \| Long-term debt
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	5,435	6,087
Fair Value, Gain	0	0
Fair Value, Loss	(1)	(1)
Not designated as hedges
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	42,127	46,400
Fair Value, Gain	183	320
Fair Value, Loss	(283)	(344)
Not designated as hedges \| Client-related and other risk management
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	27,704	27,519
Fair Value, Gain	100	210
Fair Value, Loss	(273)	(228)
Not designated as hedges \| Client-related and other risk management \| Interest rate contracts \| Pay fixed swaps
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	11,080	10,962
Fair Value, Gain	46	59
Fair Value, Loss	(22)	(155)
Not designated as hedges \| Client-related and other risk management \| Interest rate contracts \| Receive fixed swaps
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	11,214	10,880
Fair Value, Gain	41	141
Fair Value, Loss	(239)	(61)
Not designated as hedges \| Client-related and other risk management \| Interest rate contracts \| Other
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	1,233	1,658
Fair Value, Gain	4	4
Fair Value, Loss	(4)	(4)
Not designated as hedges \| Client-related and other risk management \| Interest rate contracts \| Forward commitments
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	3,589	3,549
Fair Value, Gain	6	3
Fair Value, Loss	(5)	(2)
Not designated as hedges \| Client-related and other risk management \| Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	588	470
Fair Value, Gain	3	3
Fair Value, Loss	(3)	(6)
Not designated as hedges \| Mortgage banking \| Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	3,635	4,614
Fair Value, Gain	17	12
Fair Value, Loss	(6)	(6)
Not designated as hedges \| Mortgage banking \| Interest rate contracts \| Interest rate lock commitments
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	885	1,308
Fair Value, Gain	3	7
Fair Value, Loss	(4)	(3)
Not designated as hedges \| Mortgage banking \| Interest rate contracts \| When issued securities, forward rate agreements and forward commitments
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	2,398	3,124
Fair Value, Gain	12	4
Fair Value, Loss	(2)	(3)
Not designated as hedges \| Mortgage banking \| Interest rate contracts \| Other
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	352	182
Fair Value, Gain	2	1
Fair Value, Loss	0	0
Not designated as hedges \| MSRs \| Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	10,788	14,267
Fair Value, Gain	66	98
Fair Value, Loss	(4)	(110)
Not designated as hedges \| MSRs \| Interest rate contracts \| Pay fixed swaps
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	2,674	3,418
Fair Value, Gain	0	32
Fair Value, Loss	0	(13)
Not designated as hedges \| MSRs \| Interest rate contracts \| Receive fixed swaps
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	3,901	4,498
Fair Value, Gain	0	15
Fair Value, Loss	0	(86)
Not designated as hedges \| MSRs \| Interest rate contracts \| Options
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	3,275	4,535
Fair Value, Gain	66	50
Fair Value, Loss	0	(11)
Not designated as hedges \| MSRs \| Interest rate contracts \| When issued securities, forward rate agreements and forward commitments
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	862	1,813
Fair Value, Gain	0	1
Fair Value, Loss	(4)	0
Not designated as hedges \| MSRs \| Interest rate contracts \| Other
Derivative Instruments, Gain (Loss) [Line Items]
Notional Amount	76	3
Fair Value, Gain	0	0
Fair Value, Loss	$ 0	$ 0

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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