SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Information about reportable segments [Abstract]
Net interest income	$ 244,428us-gaap_InterestIncomeExpenseNet	$ 238,094us-gaap_InterestIncomeExpenseNet	$ 230,424us-gaap_InterestIncomeExpenseNet
Provision for loan losses	7,178us-gaap_ProvisionForLoanAndLeaseLosses	7,992us-gaap_ProvisionForLoanAndLeaseLosses	9,108us-gaap_ProvisionForLoanAndLeaseLosses
Noninterest income	119,020	102,180	99,246
Amortization of intangible assets	4,287us-gaap_AmortizationOfIntangibleAssets	4,469us-gaap_AmortizationOfIntangibleAssets	4,607us-gaap_AmortizationOfIntangibleAssets
Other operating expenses	222,293us-gaap_OtherCostAndExpenseOperating	216,786us-gaap_OtherCostAndExpenseOperating	207,150us-gaap_OtherCostAndExpenseOperating
Income before income taxes	129,690	111,027	108,805
Assets	7,489,440us-gaap_Assets	7,095,864us-gaap_Assets	7,496,800us-gaap_Assets
Goodwill	375,174us-gaap_Goodwill	374,991us-gaap_Goodwill	369,703us-gaap_Goodwill
Operating Segments [Member] \| Banking [Member]
Information about reportable segments [Abstract]
Net interest income	244,243	237,915	230,251
Provision for loan losses	7,178	7,992	9,108
Noninterest income	58,565	48,030	50,422
Amortization of intangible assets	3,438	3,569	3,548
Other operating expenses	178,472	175,139	167,332
Income before income taxes	113,720	99,245	100,685
Assets	7,463,379	7,070,866	7,472,628
Goodwill	364,495	364,495	359,207
Operating Segments [Member] \| Employee Benefit Services [Member]
Information about reportable segments [Abstract]
Net interest income	92	101	104
Provision for loan losses	0	0	0
Noninterest income	43,701	39,534	36,781
Amortization of intangible assets	647	662	779
Other operating expenses	32,846	31,049	30,617
Income before income taxes	10,300	7,924	5,489
Assets	31,513	27,970	30,405
Goodwill	8,019	7,836	7,836
Operating Segments [Member] \| Wealth Management [Member]
Information about reportable segments [Abstract]
Net interest income	93	78	69
Provision for loan losses	0	0	0
Noninterest income	18,634	16,265	13,549
Amortization of intangible assets	202	238	280
Other operating expenses	12,855	12,247	10,707
Income before income taxes	5,670	3,858	2,631
Assets	15,635	13,259	12,101
Goodwill	2,660	2,660	2,660
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0
Provision for loan losses	0	0	0
Noninterest income	(1,880)	(1,649)	(1,506)
Amortization of intangible assets	0	0	0
Other operating expenses	(1,880)	(1,649)	(1,506)
Income before income taxes	0	0	0
Assets	(21,087)	(16,231)	(18,334)
Goodwill	$ 0	$ 0	$ 0

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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