Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
ASSETS
Trading account assets		$ 1,040	$ 849
Held-to-maturity securities		10,067	11,519
Held-to-maturity securities		10,116	11,122
Loans, net of unearned income		93,746	88,669
Loans held for sale, Carrying Amount	[1]	1,334	1,227
Derivative assets		780	531
Fair Value		21,843	19,428
LIABILITIES
Long-term debt, Carrying Amount		12,448	13,732
Carrying Amount [Member]
ASSETS
Trading account assets		1,040	849
Other investments		605	666
Loans, net of allowance		4	3
Held-to-maturity securities		10,067	11,519
Loans, net of unearned income		93,742	88,666
Loans held for sale, Carrying Amount		1,194	1,173
Loans held for sale		140	54
Fair Value		21,843	19,428
Cash and short-term investments		2,004	3,240
LIABILITIES
Time deposits, Carrying Amount		11,652	13,245
Short-term borrowings, Carrying Amount		1,092	863
Long-term debt, Carrying Amount		12,448	13,732
Deposits with no stated maturity		100,218	94,064
Estimate of Fair Value Measurement [Member]
ASSETS
Trading account assets		1,040	849
Other investments		605	666
Loans, net of allowance		4	3
Held-to-maturity securities		10,116	11,122
Loans, net of unearned income		92,641	86,224
Loans held for sale, Carrying Amount		1,194	1,173
Loans held for sale		140	54
Fair Value		21,843	19,428
Cash and short-term investments		2,004	3,240
LIABILITIES
Time deposits		11,752	13,331
Short-term borrowings		1,092	863
Long-term debt		12,943	13,787
Deposits with no stated maturity		100,218	94,064
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 unearned income		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		2,004	3,240
LIABILITIES
Time deposits		0	0
Short-term borrowings		19	14
Long-term debt		12,694	12,576
Deposits with no stated maturity		0	0
Estimate of Fair Value Measurement [Member] | Level 2 [Member]
ASSETS
Trading account assets		1,040	849
Other investments		0	1
Loans, net of allowance		0	0
Held-to-maturity securities		10,116	11,122
Loans, net of unearned income		0	0
Loans held for sale, Carrying Amount		0	0
Loans held for sale		140	54
Fair Value		21,832	19,408
Cash and short-term investments		0	0
LIABILITIES
Time deposits		11,752	13,331
Short-term borrowings		1,073	849
Long-term debt		249	1,211
Deposits with no stated maturity		100,218	94,064
Estimate of Fair Value Measurement [Member] | Level 3 [Member]
ASSETS
Trading account assets		0	0
Other investments		528	559
Loans, net of allowance		4	3
Held-to-maturity securities		0	0
Loans, net of unearned income		92,641	86,224
Loans held for sale, Carrying Amount		1,194	1,173
Loans held for sale		0	0
Fair Value		11	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		77	106
Not Designated as Hedging Instrument [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		715	462
LIABILITIES
Derivative liabilities		194	395
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member]
ASSETS
Derivative assets		715	462
Derivative assets, netting adjustment		(347)	(350)
LIABILITIES
Derivative liabilities		194	395
Derivative liabilities, netting adjustment		(319)	(338)
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 1 [Member]
ASSETS
Derivative assets		49	68
LIABILITIES
Derivative liabilities		43	58
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 2 [Member]
ASSETS
Derivative assets		985	736
LIABILITIES
Derivative liabilities		461	675
Not Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 3 [Member]
ASSETS
Derivative assets		28	8
LIABILITIES
Derivative liabilities		9	0
Designated as Hedging Instrument [Member] | Carrying Amount [Member]
ASSETS
Derivative assets		65	69
LIABILITIES
Derivative liabilities		4	(9)
Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member]
ASSETS
Derivative assets		65	69
Derivative assets, netting adjustment		(126)	17
LIABILITIES
Derivative liabilities		4	(9)
Derivative liabilities, netting adjustment		(16)	1
Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 1 [Member]
ASSETS
Derivative assets		0	2
LIABILITIES
Derivative liabilities		0	0
Designated as Hedging Instrument [Member] | Estimate of Fair Value Measurement [Member] | Level 2 [Member]
ASSETS
Derivative assets		191	50
LIABILITIES
Derivative liabilities		20	(10)
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 $140 million at December 31, 2019, and $54 million at December 31, 2018