SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Information about reportable segments
Net interest income	$ 107,786	$ 110,394	$ 328,095	$ 308,407
Provision for credit losses	2,878	5,061	7,130	12,005
Noninterest revenues	67,586	65,249	147,065	195,019
Loss on sales of investment securities	0	0	(52,329)	0
Gain on debt extinguishment	0	0	242	0
Other noninterest revenues			199,152
Amortization of intangible assets	3,576	3,837	10,948	11,420
Acquisition expenses	0	409	56	4,668
Acquisition-related contingent consideration adjustment	80	0	1,080	400
Other operating expenses	112,848	103,939	331,510	301,928
Income before income taxes	55,990	62,397	124,436	173,005
Assets	15,386,322	15,594,547	15,386,322	15,594,547	$ 15,835,651
Goodwill	845,396	844,984	845,396	844,984	$ 841,841
Core deposit intangibles & Other intangibles	55,938	64,240	55,938	64,240
Eliminations
Information about reportable segments
Net interest income	0	0	0	0
Provision for credit losses	0	0	0	0
Noninterest revenues	(2,077)	(1,839)		(5,773)
Loss on sales of investment securities			0
Gain on debt extinguishment			0
Other noninterest revenues			(6,425)
Amortization of intangible assets	0	0	0	0
Acquisition expenses	0	0	0	0
Acquisition-related contingent consideration adjustment	0		0	0
Other operating expenses	(2,077)	(1,839)	(6,425)	(5,773)
Income before income taxes	0	0	0	0
Assets	(115,530)	(132,141)	(115,530)	(132,141)
Goodwill	0	0	0	0
Core deposit intangibles & Other intangibles	0	0	0	0
Banking | Operating Segments
Information about reportable segments
Net interest income	107,169	110,311	326,621	308,177
Provision for credit losses	2,878	5,061	7,130	12,005
Noninterest revenues	18,448	19,427		55,634
Loss on sales of investment securities			(52,329)
Gain on debt extinguishment			242
Other noninterest revenues			54,397
Amortization of intangible assets	1,004	1,266	3,370	3,506
Acquisition expenses	0	409	16	4,665
Acquisition-related contingent consideration adjustment	0		0	0
Other operating expenses	78,884	73,554	232,647	212,616
Income before income taxes	42,851	49,448	85,768	131,019
Assets	15,160,821	15,373,021	15,160,821	15,373,021
Goodwill	732,598	735,680	732,598	735,680
Core deposit intangibles & Other intangibles	10,110	13,550	10,110	13,550
Employee Benefit Services | Operating Segments
Information about reportable segments
Net interest income	486	75	1,129	210
Provision for credit losses	0	0	0	0
Noninterest revenues	30,777	28,451		88,093
Loss on sales of investment securities			0
Gain on debt extinguishment			0
Other noninterest revenues			90,416
Amortization of intangible assets	1,596	1,633	4,860	4,974
Acquisition expenses	0	0	0	3
Acquisition-related contingent consideration adjustment	0		(100)	(100)
Other operating expenses	18,996	17,711	56,709	53,026
Income before income taxes	10,671	9,182	30,076	30,400
Assets	237,310	250,784	237,310	250,784
Goodwill	85,384	85,381	85,384	85,381
Core deposit intangibles & Other intangibles	28,258	35,045	28,258	35,045
All Other | Operating Segments
Information about reportable segments
Net interest income	131	8	345	20
Provision for credit losses	0	0	0	0
Noninterest revenues	20,438	19,210		57,065
Loss on sales of investment securities			0
Gain on debt extinguishment			0
Other noninterest revenues			60,764
Amortization of intangible assets	976	938	2,718	2,940
Acquisition expenses	0	0	40	0
Acquisition-related contingent consideration adjustment	80		1,180	500
Other operating expenses	17,045	14,513	48,579	42,059
Income before income taxes	2,468	3,767	8,592	11,586
Assets	103,721	102,883	103,721	102,883
Goodwill	27,414	23,923	27,414	23,923
Core deposit intangibles & Other intangibles	$ 17,570	$ 15,645	$ 17,570	$ 15,645