Loans and Allowance for Credit Losses (Loans, Net of Unearned Income) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 22,240,204	$ 21,212,755	$ 21,035,952
Residential Mortgages [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	2,754,388	2,990,631	3,004,958
Consumer [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,945,918	2,164,818	2,152,325
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	3,406,554	2,994,448	3,060,568
Construction and Land Development [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,096,149	1,157,451	1,190,718
Total Commercial And Industrial [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	13,037,195	11,905,407	11,627,383
Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	10,257,788	9,166,947	8,893,004
Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 2,779,407	$ 2,738,460	$ 2,734,379