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, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 294,638	$ 286,810
Total outstanding commitments	619,852	606,656
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	290,544	282,228
Total outstanding commitments	614,479	600,407
Special Mention Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,662	1,740
Total outstanding commitments	2,297	2,801
Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,432	2,842
Total outstanding commitments	3,076	3,448
Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,094	4,582
Total outstanding commitments	5,373	6,249
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	104,654	102,444
Commercial [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	102,543	100,014
Commercial [Member] | Special Mention Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,022	1,149
Commercial [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,089	1,281
Commercial [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,111	2,430
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	39,268	39,539
Commercial Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	38,287	38,473
Commercial Real Estate [Member] | Special Mention Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	625	584
Commercial Real Estate [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	356	482
Commercial Real Estate [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	981	1,066
Residential Mortgages [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	69,378	65,034
Residential Mortgages [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	68,960	64,570
Residential Mortgages [Member] | Special Mention Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3	1
Residential Mortgages [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	415	463
Residential Mortgages [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	418	464
Other Retail [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	57,448	56,430
Other Retail [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	57,142	56,101
Other Retail [Member] | Special Mention Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12	6
Other Retail [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	294	323
Other Retail [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	306	329
Credit Card [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,890	23,363
Credit Card [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	23,612	23,070
Credit Card [Member] | Classified Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	278	293
Credit Card [Member] | Total Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 278	$ 293