Asset Quality - Consumer Loans by Refreshed FICO Score (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 86,492	$ 86,038	$ 85,528
Consumer Portfolio Segment Excluding PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,355	23,724
Consumer Portfolio Segment Excluding PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	17,328	17,823
Consumer Portfolio Segment Excluding PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,784	1,781
Consumer Portfolio Segment Excluding PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,045	1,111
Consumer Portfolio Segment Excluding PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,198	3,009
Consumer Portfolio Segment, PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	391	405
Consumer Portfolio Segment, PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	386	398
Consumer Portfolio Segment, PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5	7
Consumer Portfolio Segment, PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Consumer Portfolio Segment, PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment Excluding PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,891	12,035
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10,389	9,818
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	565	498
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	430	453
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,507	1,266
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment, PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	141	133
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment, PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	140	133
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1	0
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment, PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, 750 and Above [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment Excluding PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,649	7,647
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,278	5,266
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	688	661
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	492	525
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,191	1,195
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment, PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	133	129
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment, PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	131	127
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2	2
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment, PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, 660 to 749 [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment Excluding PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,360	2,486
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,551	1,617
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	213	194
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	123	132
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	473	543
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment, PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	113	137
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment, PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	111	133
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2	4
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment, PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, Less than 660 [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, No Score [Member] | Consumer Portfolio Segment Excluding PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	455	1,556
FICO Score, No Score [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	110	1,122
FICO Score, No Score [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	318	428
FICO Score, No Score [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	1
FICO Score, No Score [Member] | Consumer Portfolio Segment Excluding PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	27	5
FICO Score, No Score [Member] | Consumer Portfolio Segment, PCI [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4	6
FICO Score, No Score [Member] | Consumer Portfolio Segment, PCI [Member] | Residential Prime Financing Receivables [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4	5
FICO Score, No Score [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Direct Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	1
FICO Score, No Score [Member] | Consumer Portfolio Segment, PCI [Member] | Credit Cards [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
FICO Score, No Score [Member] | Consumer Portfolio Segment, PCI [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0