SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jul. 12, 2019
Information about reportable segments [Abstract]
Net interest income	$ 368,403	$ 359,175	$ 345,055
Provision for credit losses	14,212	8,430	10,837
Noninterest revenue	228,419	230,619	224,059
Amortization of intangible assets	14,297	15,956	18,155
Acquisition expenses	4,933	8,608	(769)
Other operating expenses	357,304	347,462	327,903
Income before income taxes	206,076	209,338	212,988
Assets	13,931,094	11,410,295	10,607,295
Goodwill	793,708	773,810	733,503	$ 39,970
Core deposit intangibles & Other intangibles	52,940	63,113	73,846
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0
Provision for credit losses	0	0	0
Noninterest revenue	(6,214)	(3,006)	(2,993)
Amortization of intangible assets	0	0	0
Acquisition expenses	0	0	0
Other operating expenses	(6,214)	(3,006)	(2,993)
Income before income taxes	0	0	0
Assets	(131,860)	(101,255)	(66,076)
Goodwill	0	0	0
Core deposit intangibles & Other intangibles	0	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	367,237	358,334	344,551
Provision for credit losses	14,212	8,430	10,837
Noninterest revenue	69,578	75,067	75,399
Amortization of intangible assets	5,515	5,751	6,429
Acquisition expenses	4,933	8,608	(782)
Other operating expenses	255,955	245,870	231,362
Income before income taxes	156,200	164,742	172,104
Assets	13,762,325	11,225,509	10,397,623
Goodwill	690,121	670,223	629,916
Core deposit intangibles & Other intangibles	13,831	16,418	18,596
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	943	665	376
Provision for credit losses	0	0	0
Noninterest revenue	103,456	99,483	94,449
Amortization of intangible assets	5,724	6,770	8,015
Acquisition expenses	0	0	7
Other operating expenses	60,709	59,428	56,275
Income before income taxes	37,966	33,950	30,528
Assets	217,780	209,690	207,460
Goodwill	83,275	83,275	83,275
Core deposit intangibles & Other intangibles	32,051	37,775	44,545
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	223	176	128
Provision for credit losses	0	0	0
Noninterest revenue	61,599	59,075	57,204
Amortization of intangible assets	3,058	3,435	3,711
Acquisition expenses	0	0	6
Other operating expenses	46,854	45,170	43,259
Income before income taxes	11,910	10,646	10,356
Assets	82,849	76,351	68,288
Goodwill	20,312	20,312	20,312
Core deposit intangibles & Other intangibles	$ 7,058	$ 8,920	$ 10,705