Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Details) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
ASSETS
Trading account assets		$ 902	$ 735
Derivative assets		2,553	1,798
Securities available for sale		40,594	27,556
Amortized cost		8,423	7,595
Held-to-maturity securities		8,664	8,023
Loans, net of unearned income (residential)		98,609	101,185
Loans held for sale	[1]	1,805	1,583
LIABILITIES
Long-term debt		13,165	13,709
Carrying Amount
ASSETS
Trading account assets		902	735
Other investments		607	621
Loans, net of unearned income (residential)		10	11
Loans held for sale		237	264
Securities available for sale		40,594	27,556
Amortized cost		8,423	7,595
Loans, net of unearned income (residential)		97,515	99,548
Loans held for sale		1,568	1,319
Short-term investments			0
Cash and short-term investments		20,371	17,285
LIABILITIES
Time deposits		4,205	5,743
Short-term borrowings		995	979
Long-term debt		13,165	13,709
Deposits with no stated maturity		147,726	129,539
Fair Value
ASSETS
Trading account assets		902	735
Other investments		607	621
Loans, net of unearned income (residential)		10	11
Loans held for sale		237	264
Securities available for sale		40,594	27,556
Held-to-maturity securities		8,664	8,023
Loans, net of unearned income (residential)		97,435	98,946
Loans held for sale		1,568	1,319
Short-term investments			0
Cash and short-term investments		20,371	17,285
LIABILITIES
Time deposits		4,214	5,765
Short-term borrowings		995	979
Long-term debt		13,792	14,659
Deposits with no stated maturity		147,726	129,539
Level 1 | Fair Value
ASSETS
Trading account assets		17	0
Other investments		0	0
Loans, net of unearned income (residential)		0	0
Loans held for sale		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
Cash and short-term investments		20,371	17,285
LIABILITIES
Time deposits		0	0
Short-term borrowings		212	256
Long-term debt		13,076	13,925
Deposits with no stated maturity		0	0
Level 2 | Fair Value
ASSETS
Trading account assets		885	735
Other investments		0	0
Loans, net of unearned income (residential)		0	0
Loans held for sale		237	264
Securities available for sale		40,572	27,543
Held-to-maturity securities		8,664	8,023
Loans, net of unearned income (residential)		0	0
Loans held for sale		0	0
Short-term investments			0
Cash and short-term investments		0	0
LIABILITIES
Time deposits		4,214	5,765
Short-term borrowings		783	723
Long-term debt		716	734
Deposits with no stated maturity		147,726	129,539
Level 3 | Fair Value
ASSETS
Trading account assets		0	0
Other investments		537	555
Loans, net of unearned income (residential)		10	11
Loans held for sale		0	0
Securities available for sale		22	13
Held-to-maturity securities		0	0
Loans, net of unearned income (residential)		97,435	98,946
Loans held for sale		1,568	1,319
Short-term investments			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
Measured at NAV | Fair Value
ASSETS
Other investments		70	66
Derivatives Not Designated as Hedging Instruments | Carrying Amount
ASSETS
Derivative assets		2,469	1,676
LIABILITIES
Derivative liabilities		249	154
Derivatives Not Designated as Hedging Instruments | Fair Value
ASSETS
Derivative assets		2,469	1,675
Derivative assets, Netting Adjustment		(342)	(433)
LIABILITIES
Derivative liabilities		249	154
Derivative liabilities, Netting Adjustment		(1,911)	(675)
Derivatives Not Designated as Hedging Instruments | Level 1 | Fair Value
ASSETS
Derivative assets		65	78
LIABILITIES
Derivative liabilities		60	72
Derivatives Not Designated as Hedging Instruments | Level 2 | Fair Value
ASSETS
Derivative assets		2,739	1,939
LIABILITIES
Derivative liabilities		2,093	746
Derivatives Not Designated as Hedging Instruments | Level 3 | Fair Value
ASSETS
Derivative assets		7	91
LIABILITIES
Derivative liabilities		7	11
Derivatives Designated as Hedging Instruments | Carrying Amount
ASSETS
Derivative assets		84	123
LIABILITIES
Derivative liabilities		11	(3)
Derivatives Designated as Hedging Instruments | Fair Value
ASSETS
Derivative assets		84	123
Derivative assets, Netting Adjustment		39	53
LIABILITIES
Derivative liabilities		11	(3)
Derivative liabilities, Netting Adjustment		0	0
Derivatives Designated as Hedging Instruments | Level 1 | Fair Value
ASSETS
Derivative assets		0	0
LIABILITIES
Derivative liabilities		0	0
Derivatives Designated as Hedging Instruments | Level 2 | Fair Value
ASSETS
Derivative assets		45	70
LIABILITIES
Derivative liabilities		11	(3)
Derivatives Designated as Hedging Instruments | Level 3 | Fair Value
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 $237 million at September 30, 2021, and $264 million at December 31, 2020.