SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Information about reportable segments [Abstract]
Net interest income	$ 88,300	$ 86,846	$ 175,159	$ 171,470
Provision for loan losses	1,400	2,448	3,822	6,127
Noninterest revenues	60,706	56,559	116,402	114,050
Amortization of intangible assets	3,904	4,555	8,034	9,353
Acquisition expenses	1,194	71	1,728	63
Other operating expenses	86,078	81,486	170,066	163,027
Income before income taxes	56,430	54,845	107,911	106,950
Assets	10,745,388	10,633,094	10,745,388	10,633,094	$ 10,607,295
Goodwill	733,503	733,479	733,503	733,479	$ 733,503
Core deposit intangibles & Other intangibles	67,012	82,648	67,012	82,648
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0	0
Provision for loan losses	0	0	0	0
Noninterest revenues	(1,213)	(700)	(1,982)	(1,395)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	(1,213)	(700)	(1,982)	(1,395)
Income before income taxes	0	0	0	0
Assets	(68,207)	(53,663)	(68,207)	(53,663)
Goodwill	0	0	0	0
Core deposit intangibles & Other intangibles	0	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	88,127	86,723	174,842	171,253
Provision for loan losses	1,400	2,448	3,822	6,127
Noninterest revenues	21,921	20,040	39,269	40,397
Amortization of intangible assets	1,375	1,635	2,858	3,379
Acquisition expenses	1,194	65	1,728	50
Other operating expenses	60,578	57,464	120,347	114,418
Income before income taxes	45,501	45,151	85,356	87,676
Assets	10,534,555	10,415,083	10,534,555	10,415,083
Goodwill	629,916	629,916	629,916	629,916
Core deposit intangibles & Other intangibles	15,738	21,646	15,738	21,646
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	131	93	239	163
Provision for loan losses	0	0	0	0
Noninterest revenues	24,839	23,051	49,509	46,500
Amortization of intangible assets	1,669	1,989	3,438	4,077
Acquisition expenses	0	0	0	7
Other operating expenses	15,405	14,204	29,684	27,914
Income before income taxes	7,896	6,951	16,626	14,665
Assets	206,178	207,114	206,178	207,114
Goodwill	83,275	83,275	83,275	83,275
Core deposit intangibles & Other intangibles	41,107	48,484	41,107	48,484
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	42	30	78	54
Provision for loan losses	0	0	0	0
Noninterest revenues	15,159	14,168	29,606	28,548
Amortization of intangible assets	860	931	1,738	1,897
Acquisition expenses	0	6	0	6
Other operating expenses	11,308	10,518	22,017	22,090
Income before income taxes	3,033	2,743	5,929	4,609
Assets	72,862	64,560	72,862	64,560
Goodwill	20,312	20,288	20,312	20,288
Core deposit intangibles & Other intangibles	$ 10,167	$ 12,518	$ 10,167	$ 12,518