Business Segments (Details 1) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) atm branch	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business segment financial information
Federal and State Effective Tax Rate used for Segment Reporting	26.00%
Net Interest Income	$ 497,715	$ 486,352	$ 457,238
Provision for Credit Losses	16,000	13,425	16,900
Net Interest Income After Provision for Credit Losses	481,715	472,927	440,338
Noninterest Income	183,338	168,923	185,417
Noninterest Expense	(379,227)	(371,624)	(357,691)
Income Before Provision for Income Taxes	285,826	270,226	268,064
Provision for Income Taxes	(59,913)	(50,624)	(83,392)
Net Income	225,913	219,602	184,672
Total Assets	$ 18,095,496	17,143,974	17,089,052
Retail Banking
Business segment financial information
Number of branch locations through which products and services are delivered to customers | branch	68
Number of ATM's through which products and services are delivered to customers | atm	387
Net Interest Income	$ 266,429	264,459	264,041
Provision for Credit Losses	11,670	14,898	14,008
Net Interest Income After Provision for Credit Losses	254,759	249,561	250,033
Noninterest Income	86,682	79,004	85,042
Noninterest Expense	(216,688)	(211,761)	(209,807)
Income Before Provision for Income Taxes	124,753	116,804	125,268
Provision for Income Taxes	(30,725)	(29,172)	(44,545)
Net Income	94,028	87,632	80,723
Total Assets	6,732,811	6,365,263	5,936,568
Commercial Banking
Business segment financial information
Net Interest Income	185,259	179,577	171,038
Provision for Credit Losses	976	(760)	(160)
Net Interest Income After Provision for Credit Losses	184,283	180,337	171,198
Noninterest Income	33,362	23,733	21,670
Noninterest Expense	(84,616)	(81,344)	(74,209)
Income Before Provision for Income Taxes	133,029	122,726	118,659
Provision for Income Taxes	(28,852)	(28,496)	(41,797)
Net Income	104,177	94,230	76,862
Total Assets	4,254,261	3,958,523	3,742,991
Investment Services
Business segment financial information
Net Interest Income	39,374	41,222	29,693
Provision for Credit Losses	15	(61)	(21)
Net Interest Income After Provision for Credit Losses	39,359	41,283	29,714
Noninterest Income	55,696	55,338	57,105
Noninterest Expense	(64,974)	(65,847)	(61,674)
Income Before Provision for Income Taxes	30,081	30,774	25,145
Provision for Income Taxes	(7,929)	(8,113)	(9,303)
Net Income	22,152	22,661	15,842
Total Assets	321,700	349,832	336,455
Treasury and Other
Business segment financial information
Net Interest Income	6,653	1,094	(7,534)
Provision for Credit Losses	3,339	(652)	3,073
Net Interest Income After Provision for Credit Losses	3,314	1,746	(10,607)
Noninterest Income	7,598	10,848	21,600
Noninterest Expense	(12,949)	(12,672)	(12,001)
Income Before Provision for Income Taxes	(2,037)	(78)	(1,008)
Provision for Income Taxes	7,593	15,157	12,253
Net Income	5,556	15,079	11,245
Total Assets	$ 6,786,724	$ 6,470,356	$ 7,073,038