Loans and Allowance for Credit Losses - Summary of Loans by Portfolio Class and Company's Internal Credit Quality Rating (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	$ 273,846	$ 269,371
Total outstanding commitments	578,110	573,253
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	96,836	93,386
Commercial [Member] | Energy Business Sector [Member] | Special Mention or Classified Rating [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans, excluding covered loans	784	1,200
Total outstanding commitments	1,700	2,800
Trouble Debt Restructuring [Member] | Government National Mortgage Association [Member]
Financing Receivable, Recorded Investment [Line Items]
Restructured GNMA loans whose repayments are insured by the Federal Housing Administration or guaranteed by the United States Department of Veterans Affairs, were classified with a pass rating	1,800	1,600
GNMA loans 90 days or more past due whose repayments are insured by the Federal Housing Administration or guaranteed by the United States Department of Veterans Affairs, were classified with a pass rating	$ 2,100	$ 2,500