Loans and Allowance for Credit Losses - Summary of Loans by Portfolio Class and Company's Internal Credit Quality Rating (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	$ 280,061	$ 277,311
Total loans	281,461	280,432
Total outstanding commitments	601,532	591,201
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	99,273	97,561
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	39,966	40,463
Residential Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	62,904	59,783
Other Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	56,049	57,324
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	276,660	275,576
Total outstanding commitments	595,205	584,072
Pass [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	96,739	95,297
Pass [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	38,820	39,162
Pass [Member] | Residential Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	62,401	59,141
Pass [Member] | Other Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	55,732	57,009
Special Mention Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,946	1,800
Total outstanding commitments	2,810	3,142
Special Mention Criticized [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	1,382	1,130
Special Mention Criticized [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	542	648
Special Mention Criticized [Member] | Residential Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	14	16
Special Mention Criticized [Member] | Other Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	8	6
Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,855	3,056
Total outstanding commitments	3,517	3,987
Classified Criticized [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	1,152	1,134
Classified Criticized [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	604	653
Classified Criticized [Member] | Residential Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	489	626
Classified Criticized [Member] | Other Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	309	309
Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,801	4,856
Total outstanding commitments	6,327	7,129
Total Criticized [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,534	2,264
Total Criticized [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	1,146	1,301
Total Criticized [Member] | Residential Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	503	642
Total Criticized [Member] | Other Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	317	315
Credit Card [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	21,869	22,180
Credit Card [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	21,610	21,895
Credit Card [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	259	285
Credit Card [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	259	285
Total Loans, Excluding Covered Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	280,061	277,311
Total Loans, Excluding Covered Loans [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	275,302	272,504
Total Loans, Excluding Covered Loans [Member] | Special Mention Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	1,946	1,800
Total Loans, Excluding Covered Loans [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	2,813	3,007
Total Loans, Excluding Covered Loans [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	4,759	4,807
Covered Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,400	3,121
Covered Loans [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,358	3,072
Covered Loans [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	42	49
Covered Loans [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 42	$ 49