Loans and Allowance for Credit Losses (Loans, Net of Unearned Income) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 21,148,530	$ 21,789,931	$ 22,628,377
Residential Mortgages [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	2,412,459	2,665,212	2,877,316
Consumer [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,679,429	1,857,295	2,044,264
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	3,419,028	3,357,939	3,350,299
Construction and Land Development [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,295,036	1,065,057	1,128,959
Total Commercial And Industrial [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	12,342,578	12,844,428	13,227,539
Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	9,532,710	9,986,983	10,465,280
Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 2,809,868	$ 2,857,445	$ 2,762,259