SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Segment Reporting Information, Additional Information [Abstract]
Net interest income	$ 103,141	$ 92,105	$ 198,013	$ 186,059
Provision for credit losses	6,038	(4,338)	6,944	(10,057)
Noninterest revenues	64,097	59,460	129,770	117,991
Amortization of intangible assets	3,851	3,246	7,583	6,597
Acquisition expenses	3,960	4	4,259	31
Acquisition-related contingent consideration adjustment	400	0	400	0
Other operating expenses	102,213	90,293	197,989	180,161
Income before income taxes	50,776	62,360	110,608	127,318
Assets	15,487,833	14,801,287	15,487,833	14,801,287	$ 15,552,657
Goodwill	849,813	794,892	849,813	794,892	$ 799,109
Core deposit intangibles & Other intangibles	68,078	47,780	68,078	47,780
Eliminations [Member]
Segment Reporting Information, Additional Information [Abstract]
Net interest income	0	0	0	0
Provision for credit losses	0	0	0	0
Noninterest revenues	(1,871)	(1,707)	(3,934)	(3,596)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Acquisition-related contingent consideration adjustment	0		0
Other operating expenses	(1,871)	(1,707)	(3,934)	(3,596)
Income before income taxes	0	0	0	0
Assets	(121,326)	(100,699)	(121,326)	(100,699)
Goodwill	0	0	0	0
Core deposit intangibles & Other intangibles	0	0	0	0
Banking [Member] | Operating Segments [Member]
Segment Reporting Information, Additional Information [Abstract]
Net interest income	103,068	92,033	197,866	185,872
Provision for credit losses	6,038	(4,338)	6,944	(10,057)
Noninterest revenues	18,199	16,357	36,207	32,811
Amortization of intangible assets	1,195	1,240	2,240	2,564
Acquisition expenses	3,958	4	4,256	31
Acquisition-related contingent consideration adjustment	0		0
Other operating expenses	72,406	65,158	139,062	130,165
Income before income taxes	37,670	46,326	81,571	95,980
Assets	15,267,283	14,588,075	15,267,283	14,588,075
Goodwill	740,601	689,867	740,601	689,867
Core deposit intangibles & Other intangibles	14,817	11,267	14,817	11,267
Employee Benefit Services [Member] | Operating Segments [Member]
Segment Reporting Information, Additional Information [Abstract]
Net interest income	67	65	135	159
Provision for credit losses	0	0	0	0
Noninterest revenues	29,454	27,994	59,642	55,143
Amortization of intangible assets	1,670	1,327	3,341	2,681
Acquisition expenses	2	0	3	0
Acquisition-related contingent consideration adjustment	(100)		(100)
Other operating expenses	17,718	14,846	35,315	29,851
Income before income taxes	10,231	11,886	21,218	22,770
Assets	243,309	235,141	243,309	235,141
Goodwill	85,289	83,275	85,289	83,275
Core deposit intangibles & Other intangibles	36,679	29,370	36,679	29,370
All Other [Member] | Operating Segments [Member]
Segment Reporting Information, Additional Information [Abstract]
Net interest income	6	7	12	28
Provision for credit losses	0	0	0	0
Noninterest revenues	18,315	16,816	37,855	33,633
Amortization of intangible assets	986	679	2,002	1,352
Acquisition expenses	0	0	0	0
Acquisition-related contingent consideration adjustment	500		500
Other operating expenses	13,960	11,996	27,546	23,741
Income before income taxes	2,875	4,148	7,819	8,568
Assets	98,567	78,770	98,567	78,770
Goodwill	23,923	21,750	23,923	21,750
Core deposit intangibles & Other intangibles	$ 16,582	$ 7,143	$ 16,582	$ 7,143