SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 12, 2019
Information about reportable segments [Abstract]
Net interest income	$ 359,175	$ 345,055	$ 315,675
Provision for loan losses	8,430	10,837	10,984
Noninterest revenue	230,619	224,059	202,423
Amortization of intangible assets	15,956	18,155	16,941
Acquisition expenses	8,608	(769)	25,986
Other operating expenses	347,462	327,903	304,222
Income before income taxes	209,338	212,988	159,965
Assets	11,410,295	10,607,295	10,746,198
Goodwill	773,810	733,503	734,430	$ 40,307
Core deposit intangibles & Other intangibles	63,113	73,846	90,658
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0
Provision for loan losses	0	0	0
Noninterest revenue	(3,006)	(2,993)	(2,858)
Amortization of intangible assets	0	0	0
Acquisition expenses	0	0	0
Other operating expenses	(3,006)	(2,993)	(2,858)
Income before income taxes	0	0	0
Assets	(101,255)	(66,076)	(29,638)
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	358,334	344,551	315,025
Provision for loan losses	8,430	10,837	10,984
Noninterest revenue	75,067	75,399	73,337
Amortization of intangible assets	5,751	6,429	5,296
Acquisition expenses	8,608	(782)	24,549
Other operating expenses	245,870	231,362	218,608
Income before income taxes	164,742	172,104	128,925
Assets	11,225,509	10,397,623	10,505,919
Goodwill	670,223	629,916	629,916
Core deposit intangibles & Other intangibles	16,418	18,596	25,025
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	665	376	396
Provision for loan losses	0	0	0
Noninterest revenue	99,483	94,449	82,743
Amortization of intangible assets	6,770	8,015	8,578
Acquisition expenses	0	7	1,194
Other operating expenses	59,428	56,275	51,138
Income before income taxes	33,950	30,528	22,229
Assets	209,690	207,460	203,369
Goodwill	83,275	83,275	84,449
Core deposit intangibles & Other intangibles	37,775	44,545	52,288
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	176	128	254
Provision for loan losses	0	0	0
Noninterest revenue	59,075	57,204	49,201
Amortization of intangible assets	3,435	3,711	3,067
Acquisition expenses	0	6	243
Other operating expenses	45,170	43,259	37,334
Income before income taxes	10,646	10,356	8,811
Assets	76,351	68,288	66,548
Goodwill	20,312	20,312	20,065
Core deposit intangibles & Other intangibles	$ 8,920	$ 10,705	$ 13,345