SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Information about reportable segments
Net interest income	$ 437,285	$ 420,630	$ 374,412
Provision for credit losses	11,203	14,773	(8,839)
Loss on sales of investment securities	(52,329)	0	0
Noninterest revenues	214,834	258,725	246,235
Gain on debt extinguishment	242	0	0
Other noninterest revenues	266,921
Amortization of intangible assets	14,511	15,214	14,051
Acquisition expenses	63	5,021	701
Acquisition-related contingent consideration adjustment	3,280	(300)	200
Restructuring expenses	1,163	0	0
Litigation accrual	5,800	0	(100)
Other operating expenses	447,868	404,333	373,186
Income before income taxes	168,231	240,314	241,348
Assets	15,555,753	15,835,651	15,552,657
Goodwill	845,396	841,841	799,109
Core deposit intangibles & Other intangibles	52,591	60,996	65,226
Eliminations
Information about reportable segments
Net interest income	0	0	0
Provision for credit losses	0	0	0
Loss on sales of investment securities	0
Noninterest revenues		(7,837)	(7,130)
Gain on debt extinguishment	0
Other noninterest revenues	(8,607)
Amortization of intangible assets	0	0	0
Acquisition expenses	0	0	0
Acquisition-related contingent consideration adjustment	0	0	0
Restructuring expenses	0
Litigation accrual	0
Other operating expenses	(8,607)	(7,837)	(7,130)
Income before income taxes	0	0	0
Assets	(120,066)	(104,293)	(128,345)
Goodwill	0	0	0
Core deposit intangibles & Other intangibles	0	0	0
Banking | Operating Segments
Information about reportable segments
Net interest income	435,083	420,273	374,078
Provision for credit losses	11,203	14,773	(8,839)
Loss on sales of investment securities	(52,329)
Noninterest revenues		75,480	67,910
Gain on debt extinguishment	242
Other noninterest revenues	73,533
Amortization of intangible assets	4,363	4,753	4,744
Acquisition expenses	16	5,018	638
Acquisition-related contingent consideration adjustment	0	0	0
Restructuring expenses	1,163
Litigation accrual	5,800
Other operating expenses	315,024	283,942	265,525
Income before income taxes	118,960	187,267	179,920
Assets	15,323,489	15,616,885	15,325,732
Goodwill	732,598	732,088	689,868
Core deposit intangibles & Other intangibles	9,117	12,304	9,087
Employee Benefit Services | Operating Segments
Information about reportable segments
Net interest income	1,711	319	293
Provision for credit losses	0	0	0
Loss on sales of investment securities	0
Noninterest revenues		117,956	116,621
Gain on debt extinguishment	0
Other noninterest revenues	121,355
Amortization of intangible assets	6,452	6,607	6,033
Acquisition expenses	8	3	36
Acquisition-related contingent consideration adjustment	1,600	(500)	200
Restructuring expenses	0
Litigation accrual	0
Other operating expenses	75,873	71,466	64,423
Income before income taxes	39,133	40,699	46,222
Assets	245,967	226,135	257,879
Goodwill	85,384	85,384	85,321
Core deposit intangibles & Other intangibles	26,883	33,411	40,018
All Other | Operating Segments
Information about reportable segments
Net interest income	491	38	41
Provision for credit losses	0	0	0
Loss on sales of investment securities	0
Noninterest revenues		73,126	68,834
Gain on debt extinguishment	0
Other noninterest revenues	80,640
Amortization of intangible assets	3,696	3,854	3,274
Acquisition expenses	39	0	27
Acquisition-related contingent consideration adjustment	1,680	200	0
Restructuring expenses	0
Litigation accrual	0
Other operating expenses	65,578	56,762	50,368
Income before income taxes	10,138	12,348	15,206
Assets	106,363	96,924	97,391
Goodwill	27,414	24,369	23,920
Core deposit intangibles & Other intangibles	$ 16,591	$ 15,281	$ 16,121