SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Information about reportable segments [Abstract]
Net interest income	$ 248,420	$ 244,428	$ 238,094
Provision for loan losses	6,447	7,178	7,992
Noninterest income	123,299	119,020	102,180
Amortization of intangible assets	3,663	4,287	4,469
Other operating expenses	229,392	222,293	216,786
Income before income taxes	132,217	129,690	111,027
Assets	8,552,669	7,489,440	7,095,864
Goodwill	463,252	375,174	374,991
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0
Provision for loan losses	0	0	0
Noninterest income	(2,156)	(1,880)	(1,649)
Amortization of intangible assets	0	0	0
Other operating expenses	(2,156)	(1,880)	(1,649)
Income before income taxes	0	0	0
Assets	(65,637)	(21,087)	(16,231)
Goodwill	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	248,167	244,243	237,915
Provision for loan losses	6,447	7,178	7,992
Noninterest income	57,704	58,565	48,030
Amortization of intangible assets	2,803	3,438	3,569
Other operating expenses	181,865	178,472	175,139
Income before income taxes	114,756	113,720	99,245
Assets	8,513,228	7,463,379	7,070,866
Goodwill	439,052	364,495	364,495
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	132	92	101
Provision for loan losses	0	0	0
Noninterest income	46,784	43,701	39,534
Amortization of intangible assets	515	647	662
Other operating expenses	35,218	32,846	31,049
Income before income taxes	11,183	10,300	7,924
Assets	35,011	31,513	27,970
Goodwill	8,019	8,019	7,836
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	121	93	78
Provision for loan losses	0	0	0
Noninterest income	20,967	18,634	16,265
Amortization of intangible assets	345	202	238
Other operating expenses	14,465	12,855	12,247
Income before income taxes	6,278	5,670	3,858
Assets	70,067	15,635	13,259
Goodwill	$ 16,181	$ 2,660	$ 2,660