Loans and Allowance for Credit Losses, PCI, by Credit Quality Indicator (Details) - Receivables Acquired with Deteriorated Credit Quality [Member] - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	$ 263	$ 677
Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	131	237
Commercial real estate mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	119	383
Commercial Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	13	57
Pass [Member] | Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	95	402
Pass [Member] | Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	7	92
Pass [Member] | Commercial real estate mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	84	263
Pass [Member] | Commercial Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	4	47
Criticized [Member] | Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	168	275
Criticized [Member] | Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	124	145
Criticized [Member] | Commercial real estate mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	35	120
Criticized [Member] | Commercial Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
PCI loans	$ 9	$ 10