SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Jul. 12, 2019
Information about reportable segments [Abstract]
Net interest income	$ 91,951	$ 88,300	$ 182,005	$ 175,159
Provision for credit losses	9,774	1,400	15,368	3,822
Noninterest revenues	52,938	60,706	111,560	116,402
Amortization of intangible assets	3,524	3,904	7,191	8,034
Acquisition expenses	3,372	1,194	3,741	1,728
Other operating expenses	84,007	86,078	173,634	170,066
Income before income taxes	44,212	56,430	93,631	107,911
Assets	13,444,217	10,745,388	13,444,217	10,745,388	$ 11,410,295
Goodwill	792,715	733,503	792,715	733,503	$ 773,810	$ 39,970
Core deposit intangibles & Other intangibles	60,046	67,012	60,046	67,012
Eliminations [Member]
Information about reportable segments [Abstract]
Net interest income	0	0	0	0
Provision for credit losses	0	0	0	0
Noninterest revenues	(1,374)	(1,213)	(2,893)	(1,982)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Other operating expenses	(1,374)	(1,213)	(2,893)	(1,982)
Income before income taxes	0	0	0	0
Assets	(122,856)	(68,207)	(122,856)	(68,207)
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	91,645	88,127	181,409	174,842
Provision for credit losses	9,774	1,400	15,368	3,822
Noninterest revenues	14,991	21,921	33,672	39,269
Amortization of intangible assets	1,364	1,375	2,781	2,858
Acquisition expenses	3,372	1,194	3,741	1,728
Other operating expenses	59,130	60,578	123,330	120,347
Income before income taxes	32,996	45,501	69,861	85,356
Assets	13,272,929	10,534,555	13,272,929	10,534,555
Goodwill	689,128	629,916	689,128	629,916
Core deposit intangibles & Other intangibles	16,565	15,738	16,565	15,738
Employee Benefit Services [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	246	131	486	239
Provision for credit losses	0	0	0	0
Noninterest revenues	24,504	24,839	50,429	49,509
Amortization of intangible assets	1,411	1,669	2,905	3,438
Acquisition expenses	0	0	0	0
Other operating expenses	14,948	15,405	30,079	29,684
Income before income taxes	8,391	7,896	17,931	16,626
Assets	216,122	206,178	216,122	206,178
Goodwill	83,275	83,275	83,275	83,275
Core deposit intangibles & Other intangibles	34,870	41,107	34,870	41,107
All Other [Member] | Operating Segments [Member]
Information about reportable segments [Abstract]
Net interest income	60	42	110	78
Provision for credit losses	0	0	0	0
Noninterest revenues	14,817	15,159	30,352	29,606
Amortization of intangible assets	749	860	1,505	1,738
Acquisition expenses	0	0	0	0
Other operating expenses	11,303	11,308	23,118	22,017
Income before income taxes	2,825	3,033	5,839	5,929
Assets	78,022	72,862	78,022	72,862
Goodwill	20,312	20,312	20,312	20,312
Core deposit intangibles & Other intangibles	$ 8,611	$ 10,167	$ 8,611	$ 10,167