Loans and Allowance for Loan Losses (Loans, Net of Unearned Income) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 19,543,717	$ 19,004,163	$ 18,786,285
Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,846,916	2,690,472	2,596,692
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,122,528	2,115,295	2,099,294
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,311,699	2,384,599	2,511,808
Construction and Land Development [Member] | Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,523,419	1,373,421	1,373,048
Total Commercial And Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	10,739,155
Total Commercial And Industrial [Member] | Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	10,739,155	10,440,376	10,205,443
Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,438,884	8,297,937	8,129,429
Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 2,300,271	$ 2,142,439	$ 2,076,014