Loans and Allowance for Credit Losses (Loans, Net of Unearned Income) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 21,515,681	$ 21,212,755	$ 20,112,838
Residential Mortgages [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	2,979,985	2,990,631	2,933,251
Consumer [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	2,151,527	2,164,818	2,104,372
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	3,232,783	2,994,448	2,563,394
Construction and Land Development [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,098,726	1,157,451	1,340,067
Total Commercial And Industrial [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	12,052,660	11,905,407	11,171,754
Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	9,321,340	9,166,947	8,656,326
Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 2,731,320	$ 2,738,460	$ 2,515,428