Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
ASSETS
Trading account assets		$ 849	$ 836
Held-to-maturity securities		11,519	11,830
Held-to-maturity securities		11,122	11,565
Loans, net of allowance, Carrying Amount		88,669	85,528
Loans held for sale, Carrying Amount	[1]	1,227	1,107
Derivative assets		531	669
Fair Value		19,428	18,139
LIABILITIES
Long-term debt, Carrying Amount		13,732	14,333
Carrying Amount [Member]
ASSETS
Trading account assets		849	836
Other investments		666	726
Loans, net of allowance		3	2
Held-to-maturity securities		11,519	11,830
Loans, net of allowance, Carrying Amount		88,666	85,526
Loans held for sale, Carrying Amount		1,173	1,036
Loans held for sale		54	71
Fair Value		19,428	18,139
Cash and short-term investments		3,240	5,118
LIABILITIES
Time deposits, Carrying Amount		13,245	11,647
Short-term borrowings, Carrying Amount		863	1,011
Long-term debt, Carrying Amount		13,732	14,333
Deposits with no stated maturity		94,064	93,588
Estimate of Fair Value Measurement [Member]
ASSETS
Trading account assets		849	836
Other investments		666	726
Loans, net of allowance		3	2
Held-to-maturity securities		11,122	11,565
Loans, net of allowance, Carrying Amount		86,224	84,003
Loans held for sale, Carrying Amount		1,173	1,036
Loans held for sale		54	71
Fair Value		19,428	18,139
Cash and short-term investments		3,240	5,118
LIABILITIES
Time deposits		13,331	11,750
Short-term borrowings		863	1,011
Long-term debt		13,787	14,626
Deposits with no stated maturity		94,064	93,588
Estimate of Fair Value Measurement [Member] | Level 1 [Member]
ASSETS
Trading account assets		0	0
Other investments		0	0
Loans, net of allowance		0	0
Held-to-maturity securities		0	0
Loans, net of allowance, Carrying Amount		0	0
Loans held for sale, Carrying Amount		0	0
Loans held for sale		0	0
Fair Value		0	0
Cash and short-term investments		3,240	5,118
LIABILITIES
Time deposits		0	0
Short-term borrowings		14	72
Long-term debt		12,576	13,407
Deposits with no stated maturity		0	0
Estimate of Fair Value Measurement [Member] | Level 2 [Member]
ASSETS
Trading account assets		849	836
Other investments		1	4
Loans, net of allowance		0	0
Held-to-maturity securities		11,122	11,565
Loans, net of allowance, Carrying Amount		0	0
Loans held for sale, Carrying Amount		0	0
Loans held for sale		54	70
Fair Value		19,408	18,119
Cash and short-term investments		0	0
LIABILITIES
Time deposits		13,331	11,750
Short-term borrowings		849	939
Long-term debt		1,211	1,219
Deposits with no stated maturity		94,064	93,588
Estimate of Fair Value Measurement [Member] | Level 3 [Member]
ASSETS
Trading account assets		0	0
Other investments		559	598
Loans, net of allowance		3	2
Held-to-maturity securities		0	0
Loans, net of allowance, Carrying Amount		86,224	84,003
Loans held for sale, Carrying Amount		1,173	1,036
Loans held for sale		0	1
Fair Value		20	20
Cash and short-term investments		0	0
LIABILITIES
Time deposits		0	0
Short-term borrowings		0	0
Long-term debt		0	0
Deposits with no stated maturity		0	0
Estimate of Fair Value Measurement [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
ASSETS
Other investments		106	124
Fair Value, Measurements, Recurring [Member]
ASSETS
Trading account assets		824	821
Other investments		115	144
Loans held for sale		54	71
Derivative assets		531	669
Fair Value		19,428	18,139
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
ASSETS
Trading account assets		0	0
Other investments		0	0
Loans held for sale		0	0
Derivative assets		70	100
Fair Value		0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
ASSETS
Trading account assets		824	821
Other investments		1	4
Loans held for sale		54	70
Derivative assets		786	999
Fair Value		19,408	18,119
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
ASSETS
Trading account assets		0	0
Other investments		8	16
Loans held for sale		0	1
Derivative assets		8	13
Fair Value		20	20
Other Securities [Member]
ASSETS
Held-to-maturity securities		12	15
Held-to-maturity securities		12	15
Fair Value		20	20
Other Securities [Member] | Fair Value, Measurements, Recurring [Member]
ASSETS
Trading account assets		22	65
Fair Value		20	20
Other Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
ASSETS
Trading account assets		0	0
Fair Value		0	0
Other Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
ASSETS
Trading account assets		22	65
Fair Value		0	0
Other Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
ASSETS
Trading account assets		0	0
Fair Value		20	20
Not Designated as Hedging Instrument [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		462	681
LIABILITIES
Derivative liabilities		395	289
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member]
ASSETS
Derivative assets		462	681
Derivative assets, netting adjustment		(350)	(349)
LIABILITIES
Derivative liabilities		395	289
Derivative liabilities, netting adjustment		(338)	(568)
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 1 [Member]
ASSETS
Derivative assets		68	99
LIABILITIES
Derivative liabilities		58	94
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 2 [Member]
ASSETS
Derivative assets		736	918
LIABILITIES
Derivative liabilities		675	763
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 3 [Member]
ASSETS
Derivative assets		8	13
LIABILITIES
Derivative liabilities		0	0
Designated as Hedging Instrument [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		69	(12)
LIABILITIES
Derivative liabilities		(9)	2
Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member]
ASSETS
Derivative assets		69	(12)
Derivative assets, netting adjustment		17	(94)
LIABILITIES
Derivative liabilities		(9)	2
Derivative liabilities, netting adjustment		1	(48)
Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 1 [Member]
ASSETS
Derivative assets		2	1
LIABILITIES
Derivative liabilities		0	4
Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 2 [Member]
ASSETS
Derivative assets		50	81
LIABILITIES
Derivative liabilities		(10)	46
Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 3 [Member]
ASSETS
Derivative assets		0	0
LIABILITIES
Derivative liabilities		$ 0	$ 0

[1] Total loans held for sale include Real estate — residential mortgage loans held for sale at fair value of $54 million at December 31, 2018, and $71 million at December 31, 2017