Business Segments (Details 1) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) atm branch	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business segment financial information
Federal and State Effective Tax Rate used for Segment Reporting	37.00%
Net Interest Income	$ 457,238	$ 417,579	$ 394,087
Provision for Credit Losses	16,900	4,750	1,000
Net Interest Income After Provision for Credit Losses	440,338	412,829	393,087
Noninterest Income	185,417	197,343	186,219
Noninterest Expense	(357,691)	(350,578)	(348,104)
Income Before Provision for Income Taxes	268,064	259,594	231,202
Provision for Income Taxes	(83,392)	(78,133)	(70,498)
Net Income	184,672	181,461	160,704
Total Assets	$ 17,089,052	16,492,367	15,455,016
Retail Banking
Business segment financial information
Number of branch locations through which products and services are delivered to customers | branch	69
Number of A T M s through which products and services delivered to customers | atm	387
Net Interest Income	$ 264,041	242,967	202,259
Provision for Credit Losses	14,008	10,700	8,033
Net Interest Income After Provision for Credit Losses	250,033	232,267	194,226
Noninterest Income	85,042	91,824	82,391
Noninterest Expense	(209,807)	(208,389)	(199,572)
Income Before Provision for Income Taxes	125,268	115,702	77,045
Provision for Income Taxes	(44,545)	(41,067)	(27,330)
Net Income	80,723	74,635	49,715
Total Assets	5,936,568	5,342,078	4,680,888
Commercial Banking
Business segment financial information
Net Interest Income	171,038	156,080	143,944
Provision for Credit Losses	(160)	(7,322)	(1,165)
Net Interest Income After Provision for Credit Losses	171,198	163,402	145,109
Noninterest Income	21,670	26,967	22,191
Noninterest Expense	(74,209)	(70,405)	(77,500)
Income Before Provision for Income Taxes	118,659	119,964	89,800
Provision for Income Taxes	(41,797)	(42,667)	(31,375)
Net Income	76,862	77,297	58,425
Total Assets	3,742,991	3,565,912	3,099,175
Investment Services
Business segment financial information
Net Interest Income	29,693	24,714	18,494
Provision for Credit Losses	(21)	(23)	(43)
Net Interest Income After Provision for Credit Losses	29,714	24,737	18,537
Noninterest Income	57,105	57,396	58,835
Noninterest Expense	(61,674)	(59,782)	(57,852)
Income Before Provision for Income Taxes	25,145	22,351	19,520
Provision for Income Taxes	(9,303)	(8,270)	(7,222)
Net Income	15,842	14,081	12,298
Total Assets	336,455	280,410	274,469
Treasury and Other
Business segment financial information
Net Interest Income	(7,534)	(6,182)	29,390
Provision for Credit Losses	3,073	1,395	(5,825)
Net Interest Income After Provision for Credit Losses	(10,607)	(7,577)	35,215
Noninterest Income	21,600	21,156	22,802
Noninterest Expense	(12,001)	(12,002)	(13,180)
Income Before Provision for Income Taxes	(1,008)	1,577	44,837
Provision for Income Taxes	12,253	13,871	(4,571)
Net Income	11,245	15,448	40,266
Total Assets	$ 7,073,038	$ 7,303,967	$ 7,400,484