Loans and ACL - Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Provision (Benefit)	$ 148	$ 103	$ 443	$ 299
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	519	457	466	421
Charge-Offs	(23)	(16)	(105)	(62)
Recoveries	6	8	30	29
Provision (Benefit)	21	13	132	74
Ending Balance	523	462	523	462
CRE-income producing properties
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	116	141	135	162
Charge-Offs	(5)	(4)	(7)	(17)
Recoveries	3	3	7	6
Provision (Benefit)	(2)	(5)	(23)	(16)
Ending Balance	112	135	112	135
CRE-construction and development
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	28	38	37	48
Charge-Offs	(1)	(1)	(1)	(3)
Recoveries	3	3	9	9
Provision (Benefit)	(3)	(7)	(18)	(21)
Ending Balance	27	33	27	33
Dealer floor plan
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	10	10	8	10
Charge-Offs	0	0	0	0
Recoveries	0	0	0	0
Provision (Benefit)	0	(1)	2	(1)
Ending Balance	10	9	10	9
Commercial other lending subsidiaries
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	27	21	22	21
Charge-Offs	(5)	(2)	(17)	(7)
Recoveries	1	1	5	3
Provision (Benefit)	5	1	18	4
Ending Balance	28	21	28	21
Direct retail lending
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	105	113	105	110
Charge-Offs	(12)	(15)	(37)	(40)
Recoveries	7	8	20	23
Provision (Benefit)	3	10	15	23
Ending Balance	103	116	103	116
Revolving credit
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	98	102	104	110
Charge-Offs	(18)	(17)	(53)	(54)
Recoveries	5	5	15	15
Provision (Benefit)	14	11	33	30
Ending Balance	99	101	99	101
Residential mortgage-nonguaranteed
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	194	197	194	217
Charge-Offs	(11)	(7)	(26)	(26)
Recoveries	1	1	3	2
Provision (Benefit)	0	4	13	2
Ending Balance	184	195	184	195
Residential mortgage-government guaranteed
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	30	28	23	36
Charge-Offs	(2)	(3)	(4)	(4)
Recoveries	0	0	0	0
Provision (Benefit)	9	2	18	(5)
Ending Balance	37	27	37	27
Sales finance
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	36	44	40	40
Charge-Offs	(7)	(5)	(21)	(16)
Recoveries	3	2	9	7
Provision (Benefit)	4	1	8	11
Ending Balance	36	42	36	42
Retail other lending subsidiaries
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	279	249	265	235
Charge-Offs	(86)	(75)	(239)	(194)
Recoveries	11	7	31	24
Provision (Benefit)	85	76	232	192
Ending Balance	289	257	289	257
PCI
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	65	57	61	64
Charge-Offs	0	0	0	(1)
Recoveries	0	0	0	0
Provision (Benefit)	(2)	3	2	(3)
Ending Balance	63	60	63	60
ALLL
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,507	1,457	1,460	1,474
Charge-Offs	(170)	(145)	(510)	(424)
Recoveries	40	38	129	118
Provision (Benefit)	134	108	432	290
Ending Balance	1,511	1,458	1,511	1,458
RUFC
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	96	78	90	60
Charge-Offs	0	0	0	0
Recoveries	0	0	0	0
Provision (Benefit)	14	(5)	11	9
Acquisition		20	9	24
Ending Balance	110	93	110	93
ACL
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,603	1,535	1,550	1,534
Charge-Offs	(170)	(145)	(510)	(424)
Recoveries	40	38	129	118
Provision (Benefit)	148	103	443	299
Acquisition		20	9	24
Ending Balance	$ 1,621	$ 1,551	$ 1,621	$ 1,551