Segment Reporting (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Income Statement Data Abstract
Net interest income	$ 741,220	$ 707,273	$ 676,278
Noninterest income	332,680	352,883	329,357
Revenues	1,073,900	1,060,156	1,005,635
Credit provision (a)	26,000	70,000	37,500
Noninterest expense	709,133	702,560	698,347
Income before income taxes	338,767	287,596	269,788
Income tax expense (benefit)	109,503	87,322	81,487
Net income	$ 229,264	$ 200,274	$ 188,301
Return on average allocated capital (ROCET1)	10.40%	9.90%	9.90%
Segment Balance Sheet Data
Average earning assets	$ 26,999,884	$ 26,026,661	$ 24,571,087
Average loans	20,592,383	19,650,667	18,252,264
Average deposits	21,923,602	21,005,772	19,903,087
Average allocated capital (CET1)	2,109,459	1,940,391	1,833,597
Operating Segments | Corporate and Commercial Specialty
Segment Income Statement Data Abstract
Net interest income	357,051	328,603	310,072
Noninterest income	52,297	47,776	46,742
Revenues	409,348	376,379	356,814
Credit provision (a)	42,298	50,397	41,913
Noninterest expense	156,890	148,493	141,912
Income before income taxes	210,160	177,489	172,989
Income tax expense (benefit)	71,655	59,261	59,200
Net income	$ 138,505	$ 118,228	$ 113,789
Return on average allocated capital (ROCET1)	12.40%	11.00%	11.60%
Segment Balance Sheet Data
Average earning assets	$ 10,820,998	$ 10,178,813	$ 9,383,971
Average loans	10,811,827	10,169,300	9,374,191
Average deposits	6,938,913	5,904,238	5,856,530
Average allocated capital (CET1)	1,117,761	1,070,598	977,406
Operating Segments | Community, Consumer, and Business
Segment Income Statement Data Abstract
Net interest income	361,361	350,551	349,134
Noninterest income	266,250	277,942	265,503
Revenues	627,611	628,493	614,637
Credit provision (a)	20,400	24,185	25,614
Noninterest expense	490,908	502,285	492,284
Income before income taxes	116,303	102,023	96,739
Income tax expense (benefit)	40,706	35,708	33,859
Net income	$ 75,597	$ 66,315	$ 62,880
Return on average allocated capital (ROCET1)	12.90%	10.50%	9.80%
Segment Balance Sheet Data
Average earning assets	$ 9,456,549	$ 9,309,028	$ 8,810,015
Average loans	9,452,253	9,307,723	8,809,673
Average deposits	11,711,407	11,451,759	10,898,602
Average allocated capital (CET1)	586,417	629,540	640,181
Operating Segments | Risk Management and Shared Services
Segment Income Statement Data Abstract
Net interest income	22,808	28,119	17,072
Noninterest income	14,133	27,165	17,112
Revenues	36,941	55,284	34,184
Credit provision (a)	(36,698)	(4,582)	(30,027)
Noninterest expense	61,335	51,782	64,151
Income before income taxes	12,304	8,084	60
Income tax expense (benefit)	(2,858)	(7,647)	(11,572)
Net income	$ 15,162	$ 15,731	$ 11,632
Return on average allocated capital (ROCET1)	1.40%	2.80%	2.10%
Segment Balance Sheet Data
Average earning assets	$ 6,722,337	$ 6,538,820	$ 6,377,101
Average loans	328,303	173,644	68,400
Average deposits	3,273,282	3,649,775	3,147,955
Average allocated capital (CET1)	$ 405,281	$ 240,253	$ 216,010