SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jul. 12, 2019
Information about reportable segments [Abstract]
Net interest income	$ 92,965	$ 91,276	$ 274,970	$ 266,435
Provision for credit losses	1,945	1,751	17,313	5,573
Noninterest revenues	59,659	57,094	171,219	173,496
Amortization of intangible assets	3,581	3,960	10,772	11,994
Acquisition expenses	796	6,061	4,537	7,789
Other operating expenses	92,589	86,908	266,223	256,974
Income before income taxes	53,713	49,690	147,344	157,601
Assets	13,845,325	11,597,297	13,845,325	11,597,297	$ 11,410,295
Goodwill	793,749	773,610	793,749	773,610	$ 773,810	$ 39,970
Core deposit intangibles & Other intangibles	56,465	67,075	56,465	67,075
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0	0
Provision for credit losses	0	0	0	0
Noninterest revenues	(1,454)	(669)	(4,347)	(2,231)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	(1,454)	(669)	(4,347)	(2,231)
Income before income taxes	0	0	0	0
Assets	(120,877)	(76,505)	(120,877)	(76,505)
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	92,642	91,018	274,051	265,860
Provision for credit losses	1,945	1,751	17,313	5,573
Noninterest revenues	19,756	17,790	53,428	57,059
Amortization of intangible assets	1,379	1,439	4,160	4,297
Acquisition expenses	796	6,061	4,537	7,789
Other operating expenses	67,193	61,839	190,523	182,186
Income before income taxes	41,085	37,718	110,946	123,074
Assets	13,674,995	11,398,191	13,674,995	11,398,191
Goodwill	690,162	670,023	690,162	670,023
Core deposit intangibles & Other intangibles	15,186	17,872	15,186	17,872
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	256	209	742	448
Provision for credit losses	0	0	0	0
Noninterest revenues	25,651	24,830	76,080	73,919
Amortization of intangible assets	1,409	1,667	4,314	5,105
Acquisition expenses	0	0	0	0
Other operating expenses	15,078	14,721	45,157	43,985
Income before income taxes	9,420	8,651	27,351	25,277
Assets	210,679	201,442	210,679	201,442
Goodwill	83,275	83,275	83,275	83,275
Core deposit intangibles & Other intangibles	33,461	39,440	33,461	39,440
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	67	49	177	127
Provision for credit losses	0	0	0	0
Noninterest revenues	15,706	15,143	46,058	44,749
Amortization of intangible assets	793	854	2,298	2,592
Acquisition expenses	0	0	0	0
Other operating expenses	11,772	11,017	34,890	33,034
Income before income taxes	3,208	3,321	9,047	9,250
Assets	80,528	74,169	80,528	74,169
Goodwill	20,312	20,312	20,312	20,312
Core deposit intangibles & Other intangibles	$ 7,818	$ 9,763	$ 7,818	$ 9,763