Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Details) - USD ($)		Sep. 30, 2020	Dec. 31, 2019
ASSETS
Trading account assets		$ 733,000,000	$ 1,040,000,000
Loans held for sale (residential)			0
Derivative assets		1,946,000,000	780,000,000
Securities available for sale		26,895,000,000	21,843,000,000
Amortized cost		8,384,000,000	10,067,000,000
Held-to-maturity securities		8,842,000,000	10,116,000,000
Loans, net of unearned income (residential)		103,081,000,000	94,646,000,000
Loans held for sale	[1]	1,724,000,000	1,334,000,000
LIABILITIES
Long-term debt		12,685,000,000	12,448,000,000
Carrying Amount
ASSETS
Trading account assets		733,000,000	1,040,000,000
Other investments		620,000,000	605,000,000
Loans, net of unearned income (residential)		8,000,000	4,000,000
Loans held for sale (residential)		288,000,000	140,000,000
Securities available for sale		26,895,000,000	21,843,000,000
Amortized cost		8,384,000,000	10,067,000,000
Loans, net of unearned income (residential)		101,343,000,000	93,742,000,000
Loans held for sale		1,436,000,000	1,194,000,000
Short-term investments		1,000,000,000	0
Cash and short-term investments		14,104,000,000	2,004,000,000
LIABILITIES
Time deposits		6,795,000,000	11,652,000,000
Short-term borrowings		1,031,000,000	1,092,000,000
Long-term debt		12,685,000,000	12,448,000,000
Deposits with no stated maturity		129,951,000,000	100,218,000,000
Fair Value
ASSETS
Trading account assets		733,000,000	1,040,000,000
Other investments		620,000,000	605,000,000
Loans, net of unearned income (residential)		8,000,000	4,000,000
Loans held for sale (residential)		288,000,000	140,000,000
Securities available for sale		26,895,000,000	21,843,000,000
Held-to-maturity securities		8,842,000,000	10,116,000,000
Loans, net of unearned income (residential)		99,826,000,000	92,641,000,000
Loans held for sale		1,436,000,000	1,194,000,000
Short-term investments		1,000,000,000	0
Cash and short-term investments		14,104,000,000	2,004,000,000
LIABILITIES
Time deposits		6,826,000,000	11,752,000,000
Short-term borrowings		1,031,000,000	1,092,000,000
Long-term debt		13,585,000,000	12,943,000,000
Deposits with no stated maturity		129,951,000,000	100,218,000,000
Fair Value | Level 1
ASSETS
Trading account assets		1,000,000	0
Other investments		0	0
Loans, net of unearned income (residential)		0	0
Loans held for sale (residential)		0	0
Securities available for sale		0	0
Held-to-maturity securities		0	0
Loans, net of unearned income (residential)		0	0
Loans held for sale		0	0
Short-term investments		0	0
Cash and short-term investments		14,104,000,000	2,004,000,000
LIABILITIES
Time deposits		0	0
Short-term borrowings		289,000,000	19,000,000
Long-term debt		12,852,000,000	12,694,000,000
Deposits with no stated maturity		0	0
Fair Value | Level 2
ASSETS
Trading account assets		705,000,000	1,040,000,000
Other investments		0	0
Loans, net of unearned income (residential)		0	0
Loans held for sale (residential)		287,000,000	140,000,000
Securities available for sale		26,883,000,000	21,832,000,000
Held-to-maturity securities		8,842,000,000	10,116,000,000
Loans, net of unearned income (residential)		0	0
Loans held for sale		0	0
Short-term investments		1,000,000,000	0
Cash and short-term investments		0	0
LIABILITIES
Time deposits		6,826,000,000	11,752,000,000
Short-term borrowings		742,000,000	1,073,000,000
Long-term debt		733,000,000	249,000,000
Deposits with no stated maturity		129,951,000,000	100,218,000,000
Fair Value | Level 3
ASSETS
Trading account assets		27,000,000	0
Other investments		552,000,000	528,000,000
Loans, net of unearned income (residential)		8,000,000	4,000,000
Loans held for sale (residential)		1,000,000	0
Securities available for sale		12,000,000	11,000,000
Held-to-maturity securities		0	0
Loans, net of unearned income (residential)		99,826,000,000	92,641,000,000
Loans held for sale		1,436,000,000	1,194,000,000
Short-term investments		0	0
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
Fair Value | Measured at NAV
ASSETS
Other investments		68,000,000	77,000,000
Derivatives Not Designated as Hedging Instruments | Carrying Amount
ASSETS
Derivative assets		1,841,000,000	715,000,000
LIABILITIES
Derivative liabilities		166,000,000	194,000,000
Derivatives Not Designated as Hedging Instruments | Fair Value
ASSETS
Derivative assets		1,841,000,000	715,000,000
Derivative assets, Netting Adjustment		(524,000,000)	(347,000,000)
LIABILITIES
Derivative liabilities		166,000,000	194,000,000
Derivative liabilities, Netting Adjustment		(675,000,000)	(319,000,000)
Derivatives Not Designated as Hedging Instruments | Fair Value | Level 1
ASSETS
Derivative assets		56,000,000	49,000,000
LIABILITIES
Derivative liabilities		51,000,000	43,000,000
Derivatives Not Designated as Hedging Instruments | Fair Value | Level 2
ASSETS
Derivative assets		2,182,000,000	985,000,000
LIABILITIES
Derivative liabilities		775,000,000	461,000,000
Derivatives Not Designated as Hedging Instruments | Fair Value | Level 3
ASSETS
Derivative assets		127,000,000	28,000,000
LIABILITIES
Derivative liabilities		15,000,000	9,000,000
Derivatives Designated as Hedging Instruments | Carrying Amount
ASSETS
Derivative assets		105,000,000	65,000,000
LIABILITIES
Derivative liabilities		2,000,000	4,000,000
Derivatives Designated as Hedging Instruments | Fair Value
ASSETS
Derivative assets		105,000,000	65,000,000
Derivative assets, Netting Adjustment		34,000,000	(126,000,000)
LIABILITIES
Derivative liabilities		2,000,000	4,000,000
Derivative liabilities, Netting Adjustment		(4,000,000)	(16,000,000)
Derivatives Designated as Hedging Instruments | Fair Value | Level 1
ASSETS
Derivative assets		0	0
LIABILITIES
Derivative liabilities		0	0
Derivatives Designated as Hedging Instruments | Fair Value | Level 2
ASSETS
Derivative assets		71,000,000	191,000,000
LIABILITIES
Derivative liabilities		6,000,000	20,000,000
Derivatives Designated as Hedging Instruments | Fair Value | Level 3
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 $288 million at September 30, 2020, and $140 million at December 31, 2019.