SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Information about reportable segments [Abstract]
Net interest income	$ 345,055	$ 315,675	$ 273,896
Provision for loan losses	10,837	10,984	8,076
Noninterest revenue	224,059	202,423	155,625
Amortization of intangible assets	18,155	16,941	5,479
Acquisition expenses	(769)	25,986	1,706
Other operating expenses	327,903	304,222	259,663
Income before income taxes	212,988	159,965	154,597
Assets	10,607,295	10,746,198	8,666,437
Goodwill	733,503	734,430	465,142
Core deposit intangibles & Other intangibles	73,846	90,658	15,702
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0
Provision for loan losses	0	0	0
Noninterest revenue	(2,993)	(2,858)	(2,442)
Amortization of intangible assets	0	0	0
Acquisition expenses	0	0	0
Other operating expenses	(2,993)	(2,858)	(2,442)
Income before income taxes	0	0	0
Assets	(66,076)	(29,638)	(41,790)
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	344,551	315,025	273,542
Provision for loan losses	10,837	10,984	8,076
Noninterest revenue	75,399	73,337	66,059
Amortization of intangible assets	6,429	5,296	2,682
Acquisition expenses	(782)	24,549	1,005
Other operating expenses	231,362	218,608	190,263
Income before income taxes	172,104	128,925	137,575
Assets	10,397,623	10,505,919	8,598,057
Goodwill	629,916	629,916	440,870
Core deposit intangibles & Other intangibles	18,596	25,025	7,107
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	376	396	162
Provision for loan losses	0	0	0
Noninterest revenue	94,449	82,743	48,261
Amortization of intangible assets	8,015	8,578	420
Acquisition expenses	7	1,194	445
Other operating expenses	56,275	51,138	36,892
Income before income taxes	30,528	22,229	10,666
Assets	207,460	203,369	38,742
Goodwill	83,275	84,449	8,019
Core deposit intangibles & Other intangibles	44,545	52,288	666
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	128	254	192
Provision for loan losses	0	0	0
Noninterest revenue	57,204	49,201	43,747
Amortization of intangible assets	3,711	3,067	2,377
Acquisition expenses	6	243	256
Other operating expenses	43,259	37,334	34,950
Income before income taxes	10,356	8,811	6,356
Assets	68,288	66,548	71,428
Goodwill	20,312	20,065	16,253
Core deposit intangibles & Other intangibles	$ 10,705	$ 13,345	$ 7,929