Loans and Allowance for Credit Losses (Loans, Net of Unearned Income) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 21,664,859	$ 21,789,931	$ 21,515,681
Residential Mortgages [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	2,488,792	2,665,212	2,979,985
Consumer [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,756,452	1,857,295	2,151,527
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	3,411,028	3,357,939	3,232,783
Construction and Land Development [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,122,141	1,065,057	1,098,726
Total Commercial And Industrial [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	12,886,446	12,844,428	12,052,660
Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	10,091,342	9,986,983	9,321,340
Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 2,795,104	$ 2,857,445	$ 2,731,320