SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Information about reportable segments [Abstract]
Net interest income	$ 86,846	$ 78,029	$ 171,470	$ 145,303
Provision for loan losses	2,448	1,461	6,127	3,289
Noninterest revenues	56,559	51,226	114,050	95,544
Amortization of intangible assets	4,555	4,263	9,353	7,031
Acquisition expenses	71	22,896	63	24,612
Other operating expenses	81,486	75,720	163,027	144,811
Income before income taxes	54,845	24,915	106,950	61,104
Assets	10,633,094	10,884,046	10,633,094	10,884,046	$ 10,746,198
Goodwill	733,479	733,604	733,479	733,604	$ 734,430
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0	0
Provision for loan losses	0	0	0	0
Noninterest revenues	(700)	(687)	(1,395)	(1,351)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	(700)	(687)	(1,395)	(1,351)
Income before income taxes	0	0	0	0
Assets	(53,663)	(43,533)	(53,663)	(43,533)
Goodwill	0	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	86,723	77,872	171,253	145,006
Provision for loan losses	2,448	1,461	6,127	3,289
Noninterest revenues	20,040	18,062	40,397	33,929
Amortization of intangible assets	1,635	1,170	3,379	1,724
Acquisition expenses	65	22,729	50	23,250
Other operating expenses	57,464	53,890	114,418	103,383
Income before income taxes	45,151	16,684	87,676	47,289
Assets	10,415,083	10,635,177	10,415,083	10,635,177
Goodwill	629,916	631,148	629,916	631,148
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	93	94	163	172
Provision for loan losses	0	0	0	0
Noninterest revenues	23,051	21,118	46,500	38,754
Amortization of intangible assets	1,989	2,343	4,077	3,933
Acquisition expenses	0	117	7	1,179
Other operating expenses	14,204	13,068	27,914	24,438
Income before income taxes	6,951	5,684	14,665	9,376
Assets	207,114	217,020	207,114	217,020
Goodwill	83,275	84,552	83,275	84,552
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	30	63	54	125
Provision for loan losses	0	0	0	0
Noninterest revenues	14,168	12,733	28,548	24,212
Amortization of intangible assets	931	750	1,897	1,374
Acquisition expenses	6	50	6	183
Other operating expenses	10,518	9,449	22,090	18,341
Income before income taxes	2,743	2,547	4,609	4,439
Assets	64,560	75,382	64,560	75,382
Goodwill	$ 20,288	$ 17,904	$ 20,288	$ 17,904