CREDIT QUALITY ASSESSMENT - Allowance for Credit or Loan Losses by Respective Loan Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 120,865	$ 136,242	$ 136,242
Provision/ (credit) for credit losses - loans	3,331	(18,945)	(13,894)
Charge-offs	(1,620)	(171)	(2,614)
Recoveries	520	487	1,131
Net (charge-offs)/ recoveries	(1,100)	316	(1,483)
Balance at period end	123,096	117,613	120,865
Total loans	$ 11,364,284		$ 11,366,989
Allowance for credit losses on loans to total loans ratio	1.08%		1.06%
Average loans	$ 11,361,747		$ 11,354,227
Annualized net charge-offs/ (recoveries) to average loans	0.04%		0.01%
Balance of loans individually evaluated for credit loss	$ 81,115		$ 88,168
Allowance related to loans evaluated individually	$ 20,889		$ 24,000
Individual allowance to loans evaluated individually ratio	25.75%		27.22%
Contractual balance of individually evaluated loans	$ 85,363		$ 91,105
Balance of loans collectively evaluated for credit loss	11,283,169		11,278,821
Allowance related to loans evaluated collectively	$ 102,207		$ 96,865
Collective allowance to loans evaluated collectively ratio	0.91%		0.86%
Commercial investor real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Charge-offs	$ (1)		$ 0
Total loans	4,997,879		5,104,425
Commercial owner-occupied real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Charge-offs	0		0
Total loans	1,741,113		1,755,235
Commercial AD&C
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Charge-offs	0		0
Total loans	1,090,259		988,967
Residential mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Charge-offs	0		(160)
Total loans	1,511,624		1,474,521
Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Charge-offs	0		0
Total loans	97,685		121,419
Commercial real estate: | Commercial investor real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	61,439	64,737	64,737
Provision/ (credit) for credit losses - loans	2,858		(3,323)
Charge-offs	(1)		0
Recoveries	3		25
Net (charge-offs)/ recoveries	2		25
Balance at period end	64,299		61,439
Total loans	$ 4,997,879		$ 5,104,425
Allowance for credit losses on loans to total loans ratio	1.29%		1.20%
Average loans	$ 5,057,334		$ 5,133,279
Annualized net charge-offs/ (recoveries) to average loans	0.00%		0.00%
Balance of loans individually evaluated for credit loss	$ 69,000		$ 72,218
Allowance related to loans evaluated individually	$ 14,874		$ 15,353
Individual allowance to loans evaluated individually ratio	21.56%		21.26%
Contractual balance of individually evaluated loans	$ 69,831		$ 72,712
Balance of loans collectively evaluated for credit loss	4,928,879		5,032,207
Allowance related to loans evaluated collectively	$ 49,425		$ 46,086
Collective allowance to loans evaluated collectively ratio	1.00%		0.92%
Commercial real estate: | Commercial owner-occupied real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 7,536	11,646	$ 11,646
Provision/ (credit) for credit losses - loans	(656)		(4,215)
Charge-offs	0		0
Recoveries	27		105
Net (charge-offs)/ recoveries	27		105
Balance at period end	6,907		7,536
Total loans	$ 1,741,113		$ 1,755,235
Allowance for credit losses on loans to total loans ratio	0.40%		0.43%
Average loans	$ 1,746,042		$ 1,766,839
Annualized net charge-offs/ (recoveries) to average loans	(0.01%)		(0.01%)
Balance of loans individually evaluated for credit loss	$ 4,394		$ 4,640
Allowance related to loans evaluated individually	$ 1,126		$ 1,159
Individual allowance to loans evaluated individually ratio	25.63%		24.98%
Contractual balance of individually evaluated loans	$ 5,416		$ 5,623
Balance of loans collectively evaluated for credit loss	1,736,719		1,750,595
Allowance related to loans evaluated collectively	$ 5,781		$ 6,377
Collective allowance to loans evaluated collectively ratio	0.33%		0.36%
Commercial real estate: | Commercial AD&C
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 8,287	18,646	$ 18,646
Provision/ (credit) for credit losses - loans	2,275		(10,359)
Charge-offs	0		0
Recoveries	283		0
Net (charge-offs)/ recoveries	283		0
Balance at period end	10,845		8,287
Total loans	$ 1,090,259		$ 988,967
Allowance for credit losses on loans to total loans ratio	0.99%		0.84%
Average loans	$ 1,030,763		$ 1,023,669
Annualized net charge-offs/ (recoveries) to average loans	(0.11%)		0.00%
Balance of loans individually evaluated for credit loss	$ 557		$ 1,259
Allowance related to loans evaluated individually	$ 102		$ 102
Individual allowance to loans evaluated individually ratio	18.31%		8.10%
Contractual balance of individually evaluated loans	$ 581		$ 1,270
Balance of loans collectively evaluated for credit loss	1,089,702		987,708
Allowance related to loans evaluated collectively	$ 10,743		$ 8,185
Collective allowance to loans evaluated collectively ratio	0.99%		0.83%
Commercial Business
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Total loans	$ 9,338,843		$ 9,353,507
Commercial Business | Commercial business
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	31,932	28,027	28,027
Provision/ (credit) for credit losses - loans	(1,345)		4,051
Charge-offs	(1,551)		(449)
Recoveries	1		303
Net (charge-offs)/ recoveries	(1,550)		(146)
Balance at period end	29,037		31,932
Total loans	$ 1,509,592		$ 1,504,880
Allowance for credit losses on loans to total loans ratio	1.92%		2.12%
Average loans	$ 1,508,336		$ 1,440,382
Annualized net charge-offs/ (recoveries) to average loans	0.41%		0.01%
Balance of loans individually evaluated for credit loss	$ 7,164		$ 10,051
Allowance related to loans evaluated individually	$ 4,787		$ 7,386
Individual allowance to loans evaluated individually ratio	66.82%		73.49%
Contractual balance of individually evaluated loans	$ 9,535		$ 11,500
Balance of loans collectively evaluated for credit loss	1,502,428		1,494,829
Allowance related to loans evaluated collectively	$ 24,250		$ 24,546
Collective allowance to loans evaluated collectively ratio	1.61%		1.64%
Residential real estate: | Residential mortgage
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 8,890	9,424	$ 9,424
Provision/ (credit) for credit losses - loans	195		(488)
Charge-offs	0		(160)
Recoveries	6		114
Net (charge-offs)/ recoveries	6		(46)
Balance at period end	9,091		8,890
Total loans	$ 1,511,624		$ 1,474,521
Allowance for credit losses on loans to total loans ratio	0.60%		0.60%
Average loans	$ 1,491,277		$ 1,380,496
Annualized net charge-offs/ (recoveries) to average loans	0.00%		0.00%
Balance of loans individually evaluated for credit loss	$ 0		$ 0
Allowance related to loans evaluated individually	$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Contractual balance of individually evaluated loans	$ 0		$ 0
Balance of loans collectively evaluated for credit loss	1,511,624		1,474,521
Allowance related to loans evaluated collectively	$ 9,091		$ 8,890
Collective allowance to loans evaluated collectively ratio	0.60%		0.60%
Residential real estate: | Residential construction
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 729	1,337	$ 1,337
Provision/ (credit) for credit losses - loans	(306)		(608)
Charge-offs	0		0
Recoveries	0		0
Net (charge-offs)/ recoveries	0		0
Balance at period end	423		729
Total loans	$ 97,685		$ 121,419
Allowance for credit losses on loans to total loans ratio	0.43%		0.60%
Average loans	$ 110,456		$ 187,599
Annualized net charge-offs/ (recoveries) to average loans	0.00%		0.00%
Balance of loans individually evaluated for credit loss	$ 0		$ 0
Allowance related to loans evaluated individually	$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Contractual balance of individually evaluated loans	$ 0		$ 0
Balance of loans collectively evaluated for credit loss	97,685		121,419
Allowance related to loans evaluated collectively	$ 423		$ 729
Collective allowance to loans evaluated collectively ratio	0.43%		0.60%
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 2,052	$ 2,425	$ 2,425
Provision/ (credit) for credit losses - loans	310		1,048
Charge-offs	(68)		(2,005)
Recoveries	200		584
Net (charge-offs)/ recoveries	132		(1,421)
Balance at period end	2,494		2,052
Total loans	$ 416,132		$ 417,542
Allowance for credit losses on loans to total loans ratio	0.60%		0.49%
Average loans	$ 417,539		$ 421,963
Annualized net charge-offs/ (recoveries) to average loans	(0.13%)		0.34%
Balance of loans individually evaluated for credit loss	$ 0		$ 0
Allowance related to loans evaluated individually	$ 0		$ 0
Individual allowance to loans evaluated individually ratio	0.00%		0.00%
Contractual balance of individually evaluated loans	$ 0		$ 0
Balance of loans collectively evaluated for credit loss	416,132		417,542
Allowance related to loans evaluated collectively	$ 2,494		$ 2,052
Collective allowance to loans evaluated collectively ratio	0.60%		0.49%