SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Information about reportable segments
Net interest income	$ 68,306	$ 61,228	$ 135,187	$ 121,068
Provision for loan losses	2,305	591	3,646	1,214
Noninterest revenues	38,772	29,719	77,053	58,765
Amortization of intangible assets	1,403	880	2,845	1,799
Other operating expenses	64,953	55,168	131,180	110,197
Income before income taxes	38,417	34,308	74,569	66,623
Assets	8,742,111	7,908,771	8,742,111	7,908,771	$ 8,552,669
Goodwill	465,142	375,174	465,142	375,174	$ 463,252
Eliminations [Member]
Information about reportable segments
Net interest income	0	0	0	0
Provision for loan losses	0	0	0	0
Noninterest revenues	(595)	(472)	(1,161)	(936)
Amortization of intangible assets	0	0	0	0
Other operating expenses	(595)	(472)	(1,161)	(936)
Income before income taxes	0	0	0	0
Assets	(36,977)	(24,783)	(36,977)	(24,783)
Goodwill	0	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments
Net interest income	68,220	61,165	135,019	120,948
Provision for loan losses	2,305	591	3,646	1,214
Noninterest revenues	16,594	14,009	31,907	27,534
Amortization of intangible assets	691	709	1,410	1,448
Other operating expenses	47,615	43,393	96,145	87,212
Income before income taxes	34,203	30,481	65,725	58,608
Assets	8,675,329	7,882,898	8,675,329	7,882,898
Goodwill	440,870	364,495	440,870	364,495
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments
Net interest income	38	35	77	65
Provision for loan losses	0	0	0	0
Noninterest revenues	12,067	11,606	24,457	22,959
Amortization of intangible assets	114	133	230	266
Other operating expenses	9,432	8,957	18,879	17,408
Income before income taxes	2,559	2,551	5,425	5,350
Assets	34,814	33,609	34,814	33,609
Goodwill	8,018	8,019	8,018	8,019
All Other [Member]
Information about reportable segments
Net interest income	48	28	91	55
Provision for loan losses	0	0	0	0
Noninterest revenues	10,706	4,576	21,850	9,208
Amortization of intangible assets	598	38	1,205	85
Other operating expenses	8,501	3,290	17,317	6,513
Income before income taxes	1,655	1,276	3,419	2,665
Assets	68,945	17,047	68,945	17,047
Goodwill	$ 16,254	$ 2,660	$ 16,254	$ 2,660