Loans and Allowance for Credit Losses (Loans, Net of Unearned Income) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 20,886,015	$ 21,789,931	$ 22,240,204
Residential Mortgages [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	2,351,053	2,665,212	2,754,388
Consumer [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,623,116	1,857,295	1,945,918
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	3,467,939	3,357,939	3,406,554
Construction and Land Development [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,213,991	1,065,057	1,096,149
Total Commercial And Industrial [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	12,229,916	12,844,428	13,037,195
Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	9,416,990	9,986,983	10,257,788
Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 2,812,926	$ 2,857,445	$ 2,779,407