Business Segments (Business Segments Financial Information and Narrative) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) atm branch	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business segment financial information
Federal and State effective tax rate used for segment reporting			37.00%
Net Interest Income	$ 116,317	$ 103,912	$ 338,468	$ 310,486
Provision for Credit Losses	4,000	2,500	12,650	1,500
Net Interest Income After Provision for Credit Losses	112,317	101,412	325,818	308,986
Noninterest Income	42,410	48,114	143,562	150,840
Noninterest Expense	(88,598)	(87,532)	(265,355)	(260,989)
Income Before Provision for Income Taxes	66,129	61,994	204,025	198,837
Provision for Income Taxes	(20,248)	(18,501)	(62,306)	(60,889)
Net Income	45,881	43,493	141,719	137,948
Total Assets	17,268,302	16,014,643	$ 17,268,302	16,014,643	$ 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	67,128	61,747	$ 198,633	179,798
Provision for Credit Losses	3,512	2,574	10,413	7,415
Net Interest Income After Provision for Credit Losses	63,616	59,173	188,220	172,383
Noninterest Income	21,287	24,786	64,132	67,364
Noninterest Expense	(51,507)	(51,892)	(155,786)	(155,391)
Income Before Provision for Income Taxes	33,396	32,067	96,566	84,356
Provision for Income Taxes	(11,908)	(11,329)	(34,323)	(29,958)
Net Income	21,488	20,738	62,243	54,398
Total Assets	5,758,799	5,206,442	5,758,799	5,206,442
Commercial Banking
Business segment financial information
Net Interest Income	43,438	38,613	127,106	115,112
Provision for Credit Losses	(35)	(168)	(355)	(7,052)
Net Interest Income After Provision for Credit Losses	43,473	38,781	127,461	122,164
Noninterest Income	5,137	6,977	16,451	21,015
Noninterest Expense	(17,721)	(17,449)	(54,483)	(52,479)
Income Before Provision for Income Taxes	30,889	28,309	89,429	90,700
Provision for Income Taxes	(10,891)	(10,073)	(31,472)	(32,337)
Net Income	19,998	18,236	57,957	58,363
Total Assets	3,695,606	3,428,424	3,695,606	3,428,424
Investment Services and Private Banking
Business segment financial information
Net Interest Income	7,321	6,029	20,685	18,518
Provision for Credit Losses	(5)	(7)	(16)	(18)
Net Interest Income After Provision for Credit Losses	7,326	6,036	20,701	18,536
Noninterest Income	13,593	13,662	43,389	43,632
Noninterest Expense	(14,925)	(14,579)	(45,692)	(44,786)
Income Before Provision for Income Taxes	5,994	5,119	18,398	17,382
Provision for Income Taxes	(2,218)	(1,894)	(6,807)	(6,431)
Net Income	3,776	3,225	11,591	10,951
Total Assets	305,015	290,207	305,015	290,207
Treasury and Other
Business segment financial information
Net Interest Income	(1,570)	(2,477)	(7,956)	(2,942)
Provision for Credit Losses	528	101	2,608	1,155
Net Interest Income After Provision for Credit Losses	(2,098)	(2,578)	(10,564)	(4,097)
Noninterest Income	2,393	2,689	19,590	18,829
Noninterest Expense	(4,445)	(3,612)	(9,394)	(8,333)
Income Before Provision for Income Taxes	(4,150)	(3,501)	(368)	6,399
Provision for Income Taxes	4,769	4,795	10,296	7,837
Net Income	619	1,294	9,928	14,236
Total Assets	$ 7,508,882	$ 7,089,570	$ 7,508,882	$ 7,089,570