SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Information about reportable segments [Abstract]
Net interest income	$ 273,896	$ 248,420	$ 244,428
Provision for loan losses	8,076	6,447	7,178
Noninterest income	155,625	123,299	119,020
Amortization of intangible assets	5,479	3,663	4,287
Other operating expenses	261,369	229,392	222,293
Income before income taxes	154,597	132,217	129,690
Assets	8,666,437	8,552,669	7,489,440
Goodwill	465,142	463,252	375,174
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0
Provision for loan losses	0	0	0
Noninterest income	(2,442)	(2,156)	(1,880)
Amortization of intangible assets	0	0	0
Other operating expenses	(2,442)	(2,156)	(1,880)
Income before income taxes	0	0	0
Assets	(41,790)	(65,637)	(21,087)
Goodwill	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	273,542	248,167	244,243
Provision for loan losses	8,076	6,447	7,178
Noninterest income	66,059	57,704	58,565
Amortization of intangible assets	2,682	2,803	3,438
Other operating expenses	191,268	181,865	178,472
Income before income taxes	137,575	114,756	113,720
Assets	8,598,057	8,513,228	7,463,379
Goodwill	440,870	439,052	364,495
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	162	132	92
Provision for loan losses	0	0	0
Noninterest income	48,261	46,784	43,701
Amortization of intangible assets	420	515	647
Other operating expenses	37,337	35,218	32,846
Income before income taxes	10,666	11,183	10,300
Assets	38,742	35,011	31,513
Goodwill	8,019	8,019	8,019
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	192	121	93
Provision for loan losses	0	0	0
Noninterest income	43,747	20,967	18,634
Amortization of intangible assets	2,377	345	202
Other operating expenses	35,206	14,465	12,855
Income before income taxes	6,356	6,278	5,670
Assets	71,428	70,067	15,635
Goodwill	$ 16,253	$ 16,181	$ 2,660