Loans and ACL - Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses	$ 615	$ 566	$ 547
Commercial: | Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	546	522	530
Charge-Offs	(90)	(92)	(95)
Recoveries	25	39	36
Provision for credit losses	79	77	51
ACL, ending balance	560	546	522
Commercial: | CRE
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	142	118	120
Charge-Offs	(33)	(10)	(7)
Recoveries	5	3	8
Provision for credit losses	36	31	(3)
ACL, ending balance	150	142	118
Commercial: | Commercial construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	48	42	25
Charge-Offs	0	(3)	(3)
Recoveries	3	5	8
Provision for credit losses	1	4	12
ACL, ending balance	52	48	42
Commercial: | Lease financing
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	11	9	7
Charge-Offs	(11)	(4)	(5)
Recoveries	1	1	2
Provision for credit losses	9	5	5
ACL, ending balance	10	11	9
Consumer: | Residential Mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	232	209	227
Charge-Offs	(21)	(21)	(47)
Recoveries	2	2	2
Provision for credit losses	(37)	42	27
ACL, ending balance	176	232	209
Consumer: | Residential home equity and direct
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	104	113	111
Charge-Offs	(93)	(79)	(69)
Recoveries	30	25	27
Provision for credit losses	66	45	44
ACL, ending balance	107	104	113
Consumer: | Indirect auto
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	298	296	273
Charge-Offs	(370)	(342)	(350)
Recoveries	52	49	44
Provision for credit losses	324	295	329
ACL, ending balance	304	298	296
Consumer: | Indirect other
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	58	52	54
Charge-Offs	(62)	(49)	(52)
Recoveries	17	13	16
Provision for credit losses	47	42	34
ACL, ending balance	60	58	52
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	110	101	98
Charge-Offs	(109)	(76)	(68)
Recoveries	20	17	17
Provision for credit losses	101	68	54
ACL, ending balance	122	110	101
PCI
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	9	28	44
Charge-Offs	0	(2)	(1)
Recoveries	0	0	0
Provision for credit losses	(1)	(17)	(15)
ACL, ending balance	8	9	28
ALLL
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	1,558	1,490	1,489
Charge-Offs	(789)	(678)	(697)
Recoveries	155	154	160
Provision for credit losses	625	592	538
ACL, ending balance	1,549	1,558	1,490
RUFC
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	93	119	110
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision for credit losses	(10)	(26)	9
RUFC Assumed in the Merger	257
ACL, ending balance	340	93	119
ACL
Financing Receivable, Allowance for Credit Loss [Roll Forward]
ACL, beginning balance	1,651	1,609	1,599
Charge-Offs	(789)	(678)	(697)
Recoveries	155	154	160
Provision for credit losses	615	566	547
RUFC Assumed in the Merger	257
ACL, ending balance	$ 1,889	$ 1,651	$ 1,609