SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Information about reportable segments [Abstract]
Net interest income	$ 86,198	$ 84,395	$ 257,668	$ 229,698
Provision for loan losses	2,215	2,314	8,342	5,603
Noninterest revenues	55,791	52,941	169,841	148,485
Amortization of intangible assets	4,427	4,949	13,780	11,980
Acquisition expenses	(832)	580	(769)	25,192
Other operating expenses	81,638	78,247	244,665	223,058
Income before income taxes	54,541	51,246	161,491	112,350
Assets	10,659,567	10,850,218	10,659,567	10,850,218	$ 10,746,198
Goodwill	733,479	731,505	733,479	731,505	$ 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	(763)	(684)	(2,159)	(2,035)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	(763)	(684)	(2,159)	(2,035)
Income before income taxes	0	0	0	0
Assets	(53,200)	(67,461)	(53,200)	(67,461)
Goodwill	0	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	86,060	84,227	257,313	229,233
Provision for loan losses	2,215	2,314	8,342	5,603
Noninterest revenues	18,002	20,120	58,399	54,049
Amortization of intangible assets	1,535	1,796	4,914	3,520
Acquisition expenses	(832)	534	(782)	23,784
Other operating expenses	57,863	56,926	172,283	160,311
Income before income taxes	43,281	42,777	130,955	90,064
Assets	10,440,732	10,613,065	10,440,732	10,613,065
Goodwill	629,916	629,153	629,916	629,153
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	102	104	264	276
Provision for loan losses	0	0	0	0
Noninterest revenues	23,815	21,207	70,316	59,961
Amortization of intangible assets	1,970	2,323	6,047	6,256
Acquisition expenses	0	11	7	1,190
Other operating expenses	14,025	12,788	41,938	37,225
Income before income taxes	7,922	6,189	22,588	15,566
Assets	205,565	226,812	205,565	226,812
Goodwill	83,275	84,448	83,275	84,448
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	36	64	91	189
Provision for loan losses	0	0	0	0
Noninterest revenues	14,737	12,298	43,285	36,510
Amortization of intangible assets	922	830	2,819	2,204
Acquisition expenses	0	35	6	218
Other operating expenses	10,513	9,217	32,603	27,557
Income before income taxes	3,338	2,280	7,948	6,720
Assets	66,470	77,802	66,470	77,802
Goodwill	$ 20,288	$ 17,904	$ 20,288	$ 17,904