SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Information about reportable segments
Net interest income	$ 62,363	$ 61,394	$ 183,431	$ 182,672
Provision for loan losses	1,906	1,747	3,120	4,647
Noninterest revenues	31,386	31,072	90,151	89,092
Amortization of intangible assets	843	1,051	2,642	3,293
Other operating expenses	55,236	57,760	165,433	166,603
Income before income taxes	35,764	31,908	102,387	97,221
Assets	7,997,166	7,502,643	7,997,166	7,502,643	$ 7,489,440
Goodwill	375,174	375,174	375,174	375,174	$ 375,174
Eliminations [Member]
Information about reportable segments
Net interest income	0	0	0	0
Provision for loan losses	0	0	0	0
Noninterest revenues	(526)	(486)	(1,462)	(1,365)
Amortization of intangible assets	0	0	0	0
Other operating expenses	(526)	(486)	(1,462)	(1,365)
Income before income taxes	0	0	0	0
Assets	(23,058)	(17,957)	(23,058)	(17,957)
Goodwill	0	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments
Net interest income	62,295	61,350	183,243	182,537
Provision for loan losses	1,906	1,747	3,120	4,647
Noninterest revenues	15,503	15,700	43,037	43,922
Amortization of intangible assets	680	850	2,129	2,644
Other operating expenses	43,790	46,837	131,002	133,680
Income before income taxes	31,422	27,616	90,029	85,488
Assets	7,970,345	7,476,030	7,970,345	7,476,030
Goodwill	364,495	364,495	364,495	364,495
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments
Net interest income	37	21	102	67
Provision for loan losses	0	0	0	0
Noninterest revenues	11,674	11,049	34,633	32,434
Amortization of intangible assets	125	153	391	496
Other operating expenses	8,644	8,140	26,052	24,622
Income before income taxes	2,942	2,777	8,292	7,383
Assets	30,958	29,605	30,958	29,605
Goodwill	8,019	8,019	8,019	8,019
Wealth Management [Member] | Operating Segments [Member]
Information about reportable segments
Net interest income	31	23	86	68
Provision for loan losses	0	0	0	0
Noninterest revenues	4,735	4,809	13,943	14,101
Amortization of intangible assets	38	48	122	153
Other operating expenses	3,328	3,269	9,841	9,666
Income before income taxes	1,400	1,515	4,066	4,350
Assets	18,921	14,965	18,921	14,965
Goodwill	$ 2,660	$ 2,660	$ 2,660	$ 2,660