Loans and Allowance for Loan Losses (Age Analysis of Past Due Loans by Portfolio Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 291,773	$ 327,349
Current	19,251,944	18,676,814
Total loans	19,543,717	19,004,163	$ 18,786,285
Recorded Investment > 90 Days and Accruing	30,526	27,766
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	77,484	137,267
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	45,538	33,000
Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	168,751	157,082
Total Commercial And Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	172,834
Current	10,566,321
Total loans	10,739,155
Recorded Investment > 90 Days and Accruing	28,247
Total Commercial And Industrial [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12,753
Total Commercial And Industrial [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	26,913
Total Commercial And Industrial [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	133,168
Total Commercial [Member] | Commercial Real Estate - Income Producing [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,110	13,561
Current	2,303,589	2,371,038
Total loans	2,311,699	2,384,599	2,511,808
Recorded Investment > 90 Days and Accruing	1,830	489
Total Commercial [Member] | Commercial Real Estate - Income Producing [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,541	5,315
Total Commercial [Member] | Commercial Real Estate - Income Producing [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	198	2,165
Total Commercial [Member] | Commercial Real Estate - Income Producing [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,371	6,081
Total Commercial [Member] | Construction and Land Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	15,552	8,581
Current	1,507,867	1,364,840
Total loans	1,523,419	1,373,421	1,373,048
Recorded Investment > 90 Days and Accruing	78	477
Total Commercial [Member] | Construction and Land Development [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	11,976	4,113
Total Commercial [Member] | Construction and Land Development [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,532	1,056
Total Commercial [Member] | Construction and Land Development [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,044	3,412
Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		191,167
Current		10,249,209
Total loans	10,739,155	10,440,376	10,205,443
Recorded Investment > 90 Days and Accruing		24,021
Total Commercial [Member] | Total Commercial And Industrial [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		71,259
Total Commercial [Member] | Total Commercial And Industrial [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		11,409
Total Commercial [Member] | Total Commercial And Industrial [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		108,499
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	153,997	166,509
Current	8,284,887	8,131,428
Total loans	8,438,884	8,297,937	8,129,429
Recorded Investment > 90 Days and Accruing	27,606	21,989
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,752	62,766
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	26,454	10,761
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Non-Real Estate [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	116,791	92,982
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	18,837	24,658
Current	2,281,434	2,117,781
Total loans	2,300,271	2,142,439	2,076,014
Recorded Investment > 90 Days and Accruing	641	2,032
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,001	8,493
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	459	648
Total Commercial [Member] | Total Commercial And Industrial [Member] | Commercial Real Estate - Owner Occupied [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	16,377	15,517
Residential Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	66,614	74,712
Current	2,780,302	2,615,760
Total loans	2,846,916	2,690,472	2,596,692
Recorded Investment > 90 Days and Accruing		2,208
Residential Mortgages [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	36,632	33,621
Residential Mortgages [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	11,976	10,554
Residential Mortgages [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	18,006	30,537
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	28,663	39,328
Current	2,093,865	2,075,967
Total loans	2,122,528	2,115,295	$ 2,099,294
Recorded Investment > 90 Days and Accruing	371	571
Consumer [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14,582	22,959
Consumer [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,919	7,816
Consumer [Member] | Greater Than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 9,162	$ 8,553