SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 12, 2019	Dec. 31, 2018
Information about reportable segments [Abstract]
Net interest income	$ 91,276	$ 86,198	$ 266,435	$ 257,668
Provision for loan losses	1,751	2,215	5,573	8,342
Noninterest revenues	57,094	55,791	173,496	169,841
Amortization of intangible assets	3,960	4,427	11,994	13,780
Acquisition expenses	6,061	(832)	7,789	(769)
Other operating expenses	86,908	81,638	256,974	244,665
Income before income taxes	49,690	54,541	157,601	161,491
Assets	11,597,297	10,659,567	11,597,297	10,659,567		$ 10,607,295
Goodwill	773,610	733,479	773,610	733,479	$ 40,107	$ 733,503
Core deposit intangibles & Other intangibles	67,075	78,221	67,075	78,221
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0	0
Provision for loan losses	0	0	0	0
Noninterest revenues	(669)	(763)	(2,231)	(2,159)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	(669)	(763)	(2,231)	(2,159)
Income before income taxes	0	0	0	0
Assets	(76,505)	(53,200)	(76,505)	(53,200)
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	91,018	86,060	265,860	257,313
Provision for loan losses	1,751	2,215	5,573	8,342
Noninterest revenues	17,790	18,002	57,059	58,399
Amortization of intangible assets	1,439	1,535	4,297	4,914
Acquisition expenses	6,061	(832)	7,789	(782)
Other operating expenses	61,839	57,863	182,186	172,283
Income before income taxes	37,718	43,281	123,074	130,955
Assets	11,398,191	10,440,732	11,398,191	10,440,732
Goodwill	670,023	629,916	670,023	629,916
Core deposit intangibles & Other intangibles	17,872	20,111	17,872	20,111
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	209	102	448	264
Provision for loan losses	0	0	0	0
Noninterest revenues	24,830	23,815	73,919	70,316
Amortization of intangible assets	1,667	1,970	5,105	6,047
Acquisition expenses	0	0	0	7
Other operating expenses	14,721	14,025	43,985	41,938
Income before income taxes	8,651	7,922	25,277	22,588
Assets	201,442	205,565	201,442	205,565
Goodwill	83,275	83,275	83,275	83,275
Core deposit intangibles & Other intangibles	39,440	46,514	39,440	46,514
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	49	36	127	91
Provision for loan losses	0	0	0	0
Noninterest revenues	15,143	14,737	44,749	43,285
Amortization of intangible assets	854	922	2,592	2,819
Acquisition expenses	0	0	0	6
Other operating expenses	11,017	10,513	33,034	32,603
Income before income taxes	3,321	3,338	9,250	7,948
Assets	74,169	66,470	74,169	66,470
Goodwill	20,312	20,288	20,312	20,288
Core deposit intangibles & Other intangibles	$ 9,763	$ 11,596	$ 9,763	$ 11,596