Loans and Allowance for Credit Losses (Credit Quality Indicators by Segment and Portfolio Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Total Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 17,419,615	$ 17,267,424
Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	12,886,446	12,844,428
Total Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	16,609,414	16,332,902
Total Commercial [Member] | Pass [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	12,224,590	12,080,687
Total Commercial [Member] | Pass-Watch [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	462,586	541,885
Total Commercial [Member] | Pass-Watch [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	369,933	429,097
Total Commercial [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	86,154	137,592
Total Commercial [Member] | Special Mention [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	75,791	125,852
Total Commercial [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	261,461	255,045
Total Commercial [Member] | Substandard [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	216,132	208,792
Residential Mortgages [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,488,792	2,665,212
Residential Mortgages [Member] | Performing [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,445,620	2,622,422
Residential Mortgages [Member] | Nonperforming [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	43,172	42,790
Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,756,452	1,857,295
Consumer [Member] | Performing [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,733,630	1,832,885
Consumer [Member] | Nonperforming [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	22,822	24,410
Residential Mortgage and Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	4,245,244	4,522,507
Residential Mortgage and Consumer [Member] | Performing [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	4,179,250	4,455,307
Residential Mortgage and Consumer [Member] | Nonperforming [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	65,994	67,200
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	10,091,342	9,986,983
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Pass [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	9,623,336	9,439,264
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Pass-Watch [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	263,754	314,739
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Special Mention [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	56,639	79,613
Commercial Non-Real Estate [Member] | Total Commercial [Member] | Substandard [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	147,613	153,367
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,795,104	2,857,445
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Pass [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,601,254	2,641,423
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	106,179	114,358
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	19,152	46,239
Commercial Real Estate - Owner Occupied [Member] | Total Commercial [Member] | Substandard [Member] | Total Commercial And Industrial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	68,519	55,425
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	3,411,028	3,357,939
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	3,282,794	3,219,155
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Pass-Watch [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	80,597	89,968
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	4,374	5,989
Commercial Real Estate - Income Producing [Member] | Total Commercial [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	43,263	42,827
Construction and Land Development [Member] | Total Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,122,141	1,065,057
Construction and Land Development [Member] | Total Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,102,030	1,033,060
Construction and Land Development [Member] | Total Commercial [Member] | Pass-Watch [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	12,056	22,820
Construction and Land Development [Member] | Total Commercial [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	5,989	5,751
Construction and Land Development [Member] | Total Commercial [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 2,066	$ 3,426