Loans and Allowance for Credit Losses (Credit Quality Indicators by Segments and Portfolio Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Total Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 16,384,169	$ 16,057,306
Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	12,052,660	11,905,407
Total Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	15,109,069	15,014,111
Total Commercial [Member] | Pass [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	10,924,494	11,009,561
Total Commercial [Member] | Pass-Watch [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	744,969	462,502
Total Commercial [Member] | Pass-Watch [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	644,263	367,116
Total Commercial [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	85,374	101,583
Total Commercial [Member] | Special Mention [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	73,074	86,305
Total Commercial [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	429,808	479,110
Total Commercial [Member] | Substandard [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	395,880	442,425
Total Commercial [Member] | Doubtful [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	14,949
Total Commercial [Member] | Doubtful [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	14,949
Residential Mortgages [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,979,985	2,990,631
Residential Mortgages [Member] | Performing [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,936,218	2,950,854
Residential Mortgages [Member] | Nonperforming [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	43,767	39,777
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,151,527	2,164,818
Consumer [Member] | Performing [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,132,482	2,147,312
Consumer [Member] | Nonperforming [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	19,045	17,506
Residential Mortgage and Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	5,131,512	5,155,449
Residential Mortgage and Consumer [Member] | Performing [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	5,068,700	5,098,166
Residential Mortgage and Consumer [Member] | Nonperforming [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	62,812	57,283
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	9,321,340	9,166,947
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Pass [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	8,391,474	8,492,113
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Pass-Watch [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	507,455	220,850
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Special Mention [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	61,397	71,654
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Substandard [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	346,065	382,330
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Doubtful [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	14,949
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,731,320	2,738,460
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Pass [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,533,020	2,517,448
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Pass-Watch [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	136,808	146,266
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Special Mention [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	11,677	14,651
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Substandard [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	49,815	60,095
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	3,232,783	2,994,448
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	3,119,828	2,883,553
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Pass-Watch [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	75,823	69,765
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	11,330	14,995
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	25,802	26,135
Construction and Land Development [Member] | Total Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,098,726	1,157,451
Construction and Land Development [Member] | Total Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,064,747	1,120,997
Construction and Land Development [Member] | Total Commercial [Member] | Pass-Watch [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	24,883	25,621
Construction and Land Development [Member] | Total Commercial [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	970	283
Construction and Land Development [Member] | Total Commercial [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 8,126	$ 10,550