Fair Value Measurements - Carrying Amounts and Estimated Fair Values for Certain Financial Instruments (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial assets:
Cash and due from banks		$ 3,147	$ 3,278	$ 2,681	$ 2,514
Other short-term investments	[1]	33,399	1,950
Held-to-maturity securities	[2]	11	17
Loans and leases held for sale	[3]	4,741	1,400
Commercial and industrial loans		49,665	50,542
Commercial mortgage loans		10,602	10,963
Commercial construction loans		5,815	5,090
Commercial leases		2,915	3,363
Residential mortgage loans		15,928	16,724
Home equity		5,183	6,083
Indirect secured consumer loans		13,653	11,538
Credit card		2,007	2,532
Other consumer loans		3,014	2,723
Portfolio loans and leases, net		106,329	108,356
Financial liabilities:
Deposits		159,081	127,062
Federal funds purchased		300	260
Other short-term borrowings		1,192	1,011
Long-term debt	[1]	14,973	14,970
Net Carrying Amount
Financial assets:
Cash and due from banks		3,147	3,278
Other short-term investments		33,399	1,950
Other securities		524	556
Held-to-maturity securities		11	17
Loans and leases held for sale		3,260	136
Commercial and industrial loans		48,764	49,981
Commercial mortgage loans		10,200	10,876
Commercial construction loans		5,691	5,045
Commercial leases		2,886	3,346
Residential mortgage loans		15,473	16,468
Home equity		4,982	6,046
Indirect secured consumer loans		13,522	11,485
Credit card		1,755	2,364
Other consumer loans		2,895	2,683
Unallocated ALLL			(121)
Portfolio loans and leases, net		106,168	108,173
Financial liabilities:
Deposits		159,081	127,062
Federal funds purchased		300	260
Other short-term borrowings		1,192	1,011
Long-term debt		14,973	14,970
Fair Value
Financial assets:
Cash and due from banks		3,147	3,278
Other short-term investments		33,399	1,950
Other securities		524	556
Held-to-maturity securities		11	17
Loans and leases held for sale		3,269	136
Commercial and industrial loans		49,140	51,128
Commercial mortgage loans		9,968	10,823
Commercial construction loans		5,860	5,249
Commercial leases		2,842	3,133
Residential mortgage loans		16,884	17,509
Home equity		5,275	6,315
Indirect secured consumer loans		13,331	11,331
Credit card		1,934	2,774
Other consumer loans		3,098	2,866
Unallocated ALLL			0
Portfolio loans and leases, net		108,332	111,128
Financial liabilities:
Deposits		159,094	127,059
Federal funds purchased		300	260
Other short-term borrowings		1,192	1,011
Long-term debt		16,529	15,944
Fair Value | Level 1
Financial assets:
Cash and due from banks		3,147	3,278
Other short-term investments		33,399	1,950
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
Unallocated ALLL			0
Portfolio loans and leases, net		0	0
Financial liabilities:
Deposits		0	0
Federal funds purchased		300	260
Other short-term borrowings		0	0
Long-term debt		15,606	15,244
Fair Value | Level 2
Financial assets:
Cash and due from banks		0	0
Other short-term investments		0	0
Other securities		524	556
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
Unallocated ALLL			0
Portfolio loans and leases, net		0	0
Financial liabilities:
Deposits		159,094	127,059
Federal funds purchased		0	0
Other short-term borrowings		1,192	1,011
Long-term debt		923	700
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		11	17
Loans and leases held for sale		3,269	136
Commercial and industrial loans		49,140	51,128
Commercial mortgage loans		9,968	10,823
Commercial construction loans		5,860	5,249
Commercial leases		2,842	3,133
Residential mortgage loans		16,884	17,509
Home equity		5,275	6,315
Indirect secured consumer loans		13,331	11,331
Credit card		1,934	2,774
Other consumer loans		3,098	2,866
Unallocated ALLL			0
Portfolio loans and leases, net		108,332	111,128
Financial liabilities:
Deposits		0	0
Federal funds purchased		0	0
Other short-term borrowings		0	0
Long-term debt		$ 0	$ 0

[1] Includes $55 and $74 of other short-term investments, $756 and $1,354 of portfolio loans and leases, $(7) and $(7) of ALLL, $5 and $8 of other assets, $2 and $2 of other liabilities and $656 and $1,253 of long-term debt from consolidated VIEs that are included in their respective captions above at December 31, 2020 and 2019, respectively. For further information, refer to Note 13.

[2] Fair value of $11 and $17 at December 31, 2020 and 2019, respectively.

[3] Includes $1,481 and $1,264 of residential mortgage loans held for sale measured at fair value at December 31, 2020 and 2019, respectively.