Fair Value Measurements - Carrying Amounts and Estimated Fair Values for Certain Financial Instruments (Details) - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020
Financial assets:
Cash and due from banks		$ 3,122	$ 3,147
Other short-term investments	[1]	34,187	33,399
Held-to-maturity securities	[2]	10	11
Loans and leases held for sale	[3]	5,477	4,741
Commercial and industrial loans		49,094	49,665
Commercial mortgage loans		10,481	10,602
Commercial construction loans		6,198	5,815
Commercial leases		3,255	2,915
Residential mortgage loans		15,776	15,928
Home equity		4,815	5,183
Indirect secured consumer loans		14,336	13,653
Credit card		1,810	2,007
Other consumer loans		3,090	3,014
Portfolio loans and leases, net		106,647	106,329
Financial liabilities:
Deposits		162,393	159,081
Federal funds purchased		302	300
Other short-term borrowings		1,106	1,192
Long-term debt	[1]	14,743	14,973
Net Carrying Amount
Financial assets:
Cash and due from banks		3,122	3,147
Other short-term investments		34,187	33,399
Other securities		521	524
Held-to-maturity securities		10	11
Loans and leases held for sale		3,667	3,260
Commercial and industrial loans		48,289	48,764
Commercial mortgage loans		10,092	10,200
Commercial construction loans		6,087	5,691
Commercial leases		3,231	2,886
Residential mortgage loans		15,376	15,473
Home equity		4,650	4,982
Indirect secured consumer loans		14,224	13,522
Credit card		1,584	1,755
Other consumer loans		2,961	2,895
Portfolio loans and leases, net		106,494	106,168
Financial liabilities:
Deposits		162,393	159,081
Federal funds purchased		302	300
Other short-term borrowings		1,106	1,192
Long-term debt		14,743	14,973
Total Fair Value
Financial assets:
Cash and due from banks		3,122	3,147
Other short-term investments		34,187	33,399
Other securities		521	524
Held-to-maturity securities		10	11
Loans and leases held for sale		3,677	3,269
Commercial and industrial loans		48,868	49,140
Commercial mortgage loans		9,851	9,968
Commercial construction loans		6,266	5,860
Commercial leases		3,197	2,842
Residential mortgage loans		16,338	16,884
Home equity		4,894	5,275
Indirect secured consumer loans		13,960	13,331
Credit card		1,753	1,934
Other consumer loans		3,180	3,098
Portfolio loans and leases, net		108,307	108,332
Financial liabilities:
Deposits		162,398	159,094
Federal funds purchased		302	300
Other short-term borrowings		1,106	1,192
Long-term debt		15,998	16,529
Total Fair Value | Level 1
Financial assets:
Cash and due from banks		3,122	3,147
Other short-term investments		34,187	33,399
Other securities		0	0
Held-to-maturity securities		0	0
Loans and leases held for sale		0	0
Commercial and industrial loans		0	0
Commercial mortgage loans		0	0
Commercial construction loans		0	0
Commercial leases		0	0
Residential mortgage loans		0	0
Home equity		0	0
Indirect secured consumer loans		0	0
Credit card		0	0
Other consumer loans		0	0
Portfolio loans and leases, net		0	0
Financial liabilities:
Deposits		0	0
Federal funds purchased		302	300
Other short-term borrowings		0	0
Long-term debt		15,200	15,606
Total Fair Value | Level 2
Financial assets:
Cash and due from banks		0	0
Other short-term investments		0	0
Other securities		521	524
Held-to-maturity securities		0	0
Loans and leases held for sale		0	0
Commercial and industrial loans		0	0
Commercial mortgage loans		0	0
Commercial construction loans		0	0
Commercial leases		0	0
Residential mortgage loans		0	0
Home equity		0	0
Indirect secured consumer loans		0	0
Credit card		0	0
Other consumer loans		0	0
Portfolio loans and leases, net		0	0
Financial liabilities:
Deposits		162,398	159,094
Federal funds purchased		0	0
Other short-term borrowings		1,106	1,192
Long-term debt		798	923
Total Fair Value | Level 3
Financial assets:
Cash and due from banks		0	0
Other short-term investments		0	0
Other securities		0	0
Held-to-maturity securities		10	11
Loans and leases held for sale		3,677	3,269
Commercial and industrial loans		48,868	49,140
Commercial mortgage loans		9,851	9,968
Commercial construction loans		6,266	5,860
Commercial leases		3,197	2,842
Residential mortgage loans		16,338	16,884
Home equity		4,894	5,275
Indirect secured consumer loans		13,960	13,331
Credit card		1,753	1,934
Other consumer loans		3,180	3,098
Portfolio loans and leases, net		108,307	108,332
Financial liabilities:
Deposits		0	0
Federal funds purchased		0	0
Other short-term borrowings		0	0
Long-term debt		$ 0	$ 0

[1] Includes $54 and $55 of other short-term investments, $633 and $756 of portfolio loans and leases, $(5) and $(7) of ALLL, $4 and $5 of other assets, $1 and $2 of other liabilities, and $540 and $656 of long-term debt from consolidated VIEs that are included in their respective captions above at March 31, 2021 and December 31, 2020, respectively. For further information, refer to Note 11.

[2] Fair value of $10 and $11 at March 31, 2021 and December 31, 2020, respectively.

[3] Includes $1,801 and $1,481 of residential mortgage loans held for sale measured at fair value and $9 and $0 of commercial loans held for sale measured at fair value at March 31, 2021 and December 31, 2020, respectively.