Business Segments (Business Segments Financial Information and Narrative) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) atm branch	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business segment financial information
Federal and State effective tax rate used for segment reporting			37.00%
Net Interest Income	$ 112,279	$ 103,550	$ 222,151	$ 206,574
Provision for Credit Losses	4,250	1,000	8,650	(1,000)
Net Interest Income After Provision for Credit Losses	108,029	102,550	213,501	207,574
Noninterest Income	45,236	46,519	101,152	102,726
Noninterest Expense	(88,189)	(86,071)	(176,757)	(173,457)
Income Before Provision for Income Taxes	65,076	62,998	137,896	136,843
Provision for Income Taxes	(20,414)	(18,753)	(42,058)	(42,388)
Net Income	44,662	44,245	95,838	94,455
Total Assets	16,981,292	15,860,901	$ 16,981,292	15,860,901	$ 16,492,367
Retail Banking
Business segment financial information
Number of branch locations through which products and services are delivered to customers | branch			69
Number of ATM's through which products and services are delivered to customers | atm			388
Net Interest Income	66,348	60,041	$ 131,505	118,051
Provision for Credit Losses	3,099	2,006	6,900	4,842
Net Interest Income After Provision for Credit Losses	63,249	58,035	124,605	113,209
Noninterest Income	21,920	21,771	42,845	42,577
Noninterest Expense	(52,018)	(50,758)	(104,278)	(103,498)
Income Before Provision for Income Taxes	33,151	29,048	63,172	52,288
Provision for Income Taxes	(11,741)	(10,402)	(22,415)	(18,629)
Net Income	21,410	18,646	40,757	33,659
Total Assets	5,626,767	5,076,204	5,626,767	5,076,204
Commercial Banking
Business segment financial information
Net Interest Income	41,737	38,151	83,668	76,499
Provision for Credit Losses	(132)	(258)	(320)	(6,883)
Net Interest Income After Provision for Credit Losses	41,869	38,409	83,988	83,382
Noninterest Income	5,876	6,438	11,314	14,038
Noninterest Expense	(18,407)	(17,762)	(36,762)	(35,029)
Income Before Provision for Income Taxes	29,338	27,085	58,540	62,391
Provision for Income Taxes	(10,325)	(9,608)	(20,581)	(22,264)
Net Income	19,013	17,477	37,959	40,127
Total Assets	3,658,867	3,239,572	3,658,867	3,239,572
Investment Services
Business segment financial information
Net Interest Income	6,714	6,037	13,364	12,489
Provision for Credit Losses	(6)	(5)	(11)	(11)
Net Interest Income After Provision for Credit Losses	6,720	6,042	13,375	12,500
Noninterest Income	15,247	15,946	29,796	29,971
Noninterest Expense	(15,295)	(14,780)	(30,766)	(30,207)
Income Before Provision for Income Taxes	6,672	7,208	12,405	12,264
Provision for Income Taxes	(2,469)	(2,667)	(4,590)	(4,537)
Net Income	4,203	4,541	7,815	7,727
Total Assets	307,529	282,143	307,529	282,143
Treasury and Other
Business segment financial information
Net Interest Income	(2,520)	(679)	(6,386)	(465)
Provision for Credit Losses	1,289	(743)	2,081	1,052
Net Interest Income After Provision for Credit Losses	(3,809)	64	(8,467)	(1,517)
Noninterest Income	2,193	2,364	17,197	16,140
Noninterest Expense	(2,469)	(2,771)	(4,951)	(4,723)
Income Before Provision for Income Taxes	(4,085)	(343)	3,779	9,900
Provision for Income Taxes	4,121	3,924	5,528	3,042
Net Income	36	3,581	9,307	12,942
Total Assets	$ 7,388,129	$ 7,262,982	$ 7,388,129	$ 7,262,982