Loans and ACL - Analysis of the Allowance for Credit Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Provision (Benefit)	$ 547	$ 572	$ 428
Commercial | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	530	488	446
Charge-Offs	(95)	(143)	(90)
Recoveries	36	44	40
Provision (Benefit)	51	141	92
Other	0	0	0
Ending Balance	522	530	488
Commercial | CRE
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	145	175	212
Charge-Offs	(10)	(9)	(24)
Recoveries	16	19	18
Provision (Benefit)	9	(40)	(31)
Other	0	0	0
Ending Balance	160	145	175
Commercial | Lease financing
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	7	5	4
Charge-Offs	(5)	(6)	0
Recoveries	2	2	0
Provision (Benefit)	5	6	1
Other	0	0	0
Ending Balance	9	7	5
Retail | Residential mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	227	217	253
Charge-Offs	(47)	(40)	(46)
Recoveries	2	3	3
Provision (Benefit)	27	47	7
Other	0	0	0
Ending Balance	209	227	217
Retail | Direct
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	103	105	110
Charge-Offs	(61)	(53)	(54)
Recoveries	25	26	29
Provision (Benefit)	39	25	20
Other	0	0	0
Ending Balance	106	103	105
Retail | Indirect
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	327	305	275
Charge-Offs	(402)	(366)	(303)
Recoveries	60	55	42
Provision (Benefit)	363	333	291
Other	0	0	0
Ending Balance	348	327	305
Revolving credit
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	106	104	110
Charge-Offs	(76)	(69)	(70)
Recoveries	19	20	20
Provision (Benefit)	59	51	44
Other	0	0	0
Ending Balance	108	106	104
PCI
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	44	61	64
Charge-Offs	(1)	(15)	(1)
Recoveries	0	0	0
Provision (Benefit)	(15)	(2)	(2)
Other	0	0	0
Ending Balance	28	44	61
ALLL
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,489	1,460	1,474
Charge-Offs	(697)	(701)	(588)
Recoveries	160	169	152
Provision (Benefit)	538	561	422
Other	0	0	0
Ending Balance	1,490	1,489	1,460
RUFC
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	110	90	60
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision (Benefit)	9	11	6
Other	0	9	24
Ending Balance	119	110	90
ACL
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,599	1,550	1,534
Charge-Offs	(697)	(701)	(588)
Recoveries	160	169	152
Provision (Benefit)	547	572	428
Other	0	9	24
Ending Balance	$ 1,609	$ 1,599	$ 1,550