SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Information about reportable segments [Abstract]
Net interest income	$ 86,859	$ 84,624
Provision for loan losses	2,422	3,679
Noninterest revenues	55,696	57,491
Amortization of intangible assets	4,130	4,798
Acquisition expenses	534	(8)
Other operating expenses	83,988	81,541
Income before income taxes	51,481	52,105
Assets	10,916,467	10,966,555	$ 10,607,295
Goodwill	733,503	733,625	$ 733,503
Core deposit intangibles & Other intangibles	70,916	86,959
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0
Provision for loan losses	0	0
Noninterest revenues	(769)	(695)
Amortization of intangible assets	0	0
Acquisition expenses	0	0
Other operating expenses	(769)	(695)
Income before income taxes	0	0
Assets	(53,309)	(43,539)
Goodwill	0	0
Core deposit intangibles & Other intangibles	0	0
Banking [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	86,715	84,530
Provision for loan losses	2,422	3,679
Noninterest revenues	17,348	20,357
Amortization of intangible assets	1,483	1,744
Acquisition expenses	534	(15)
Other operating expenses	59,769	56,955
Income before income taxes	39,855	42,524
Assets	10,699,384	10,736,383
Goodwill	629,916	629,916
Core deposit intangibles & Other intangibles	17,113	23,281
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	108	70
Provision for loan losses	0	0
Noninterest revenues	24,670	23,449
Amortization of intangible assets	1,769	2,088
Acquisition expenses	0	7
Other operating expenses	14,279	13,709
Income before income taxes	8,730	7,715
Assets	199,345	205,544
Goodwill	83,275	83,275
Core deposit intangibles & Other intangibles	42,777	50,200
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	36	24
Provision for loan losses	0	0
Noninterest revenues	14,447	14,380
Amortization of intangible assets	878	966
Acquisition expenses	0	0
Other operating expenses	10,709	11,572
Income before income taxes	2,896	1,866
Assets	71,047	68,167
Goodwill	20,312	20,434
Core deposit intangibles & Other intangibles	$ 11,026	$ 13,478