LOANS AND ALLOWANCE FOR CREDIT LOSSES - Amount of Business Lending Loans by Credit Quality Categories (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Financing Receivable, Recorded Investment [Line Items]
Provision for credit losses	$ 6,148	$ 3,500
Total Loans	9,883,500		$ 9,704,598
Year One writeoff			0
Year Two writeoff			160
Year Three writeoff			0
Year Four writeoff			0
Year Five writeoff			0
Prior			36
Revolving Loans Amortized Cost Basis			588
Revolving Loans Converted to Term			0
Total Loans			784
Legacy Loan
Financing Receivable, Recorded Investment [Line Items]
Total Loans	9,883,500		9,704,598
Business lending
Financing Receivable, Recorded Investment [Line Items]
2023	124,991		472,811
2022	451,091		638,783
2021	628,483		337,778
2020	329,523		196,592
2019	186,794		224,483
Prior	947,292		757,308
Revolving Loans Amortized Cost Basis	862,079		804,088
Revolving Loans Converted to Term	689,946		652,553
Total Loans	4,220,199		4,084,396
Year One writeoff	0
Year Two writeoff	64
Year Three writeoff	0
Year Four writeoff	0
Year Five writeoff	0
Prior	0
Revolving Loans Amortized Cost Basis	195
Revolving Loans Converted to Term	2
Total Loans	261
Business lending | Pass
Financing Receivable, Recorded Investment [Line Items]
2023	124,238		439,165
2022	418,013		624,181
2021	612,153		316,926
2020	306,294		187,579
2019	181,208		217,713
Prior	858,656		679,615
Revolving Loans Amortized Cost Basis	809,365		756,207
Revolving Loans Converted to Term	620,464		585,802
Total Loans	3,930,391		3,807,188
Business lending | Special Mention
Financing Receivable, Recorded Investment [Line Items]
2023	739		31,781
2022	31,221		12,599
2021	12,579		18,249
2020	20,110		3,572
2019	3,395		4,134
Prior	54,643		46,986
Revolving Loans Amortized Cost Basis	36,332		35,890
Revolving Loans Converted to Term	19,086		25,904
Total Loans	178,105		179,115
Business lending | Substandard
Financing Receivable, Recorded Investment [Line Items]
2023	14		1,865
2022	1,857		2,003
2021	3,751		2,603
2020	2,153		4,953
2019	2,191		2,636
Prior	33,993		30,707
Revolving Loans Amortized Cost Basis	16,382		11,991
Revolving Loans Converted to Term	50,396		40,847
Total Loans	110,737		97,605
Business lending | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	0		0
2021	0		0
2020	966		488
2019	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	966		488
CRE - multifamily
Financing Receivable, Recorded Investment [Line Items]
2023	4,033		104,063
2022	103,725		153,613
2021	152,612		52,058
2020	54,624		20,047
2019	19,362		42,543
Prior	155,357		117,102
Revolving Loans Amortized Cost Basis	27,430		3,387
Revolving Loans Converted to Term	141,135		126,981
Total Loans	658,278		619,794
Year One writeoff	0		0
Year Two writeoff	0		0
Year Three writeoff	0		0
Year Four writeoff	0		0
Year Five writeoff	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	0		0
CRE - multifamily | Pass
Financing Receivable, Recorded Investment [Line Items]
2023	4,033		90,888
2022	90,550		145,337
2021	145,314		52,058
2020	54,120		19,982
2019	19,298		41,992
Prior	148,210		112,287
Revolving Loans Amortized Cost Basis	27,281		3,237
Revolving Loans Converted to Term	120,765		106,580
Total Loans	609,571		572,361
CRE - multifamily | Special Mention
Financing Receivable, Recorded Investment [Line Items]
2023	0		13,175
2022	13,175		7,317
2021	7,298		0
2020	0		65
2019	64		0
Prior	5,322		3,522
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	8,258		8,289
Total Loans	34,117		32,368
CRE - multifamily | Substandard
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	0		959
2021	0		0
2020	504		0
2019	0		551
Prior	1,825		1,293
Revolving Loans Amortized Cost Basis	149		150
Revolving Loans Converted to Term	12,112		12,112
Total Loans	14,590		15,065
CRE - multifamily | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2019	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	0		0
CRE - owner occupied
Financing Receivable, Recorded Investment [Line Items]
2023	24,861		62,682
2022	62,010		92,108
2021	91,027		60,369
2020	59,292		48,584
2019	47,791		82,347
Prior	283,159		211,838
Revolving Loans Amortized Cost Basis	28,697		29,707
Revolving Loans Converted to Term	180,465		165,139
Total Loans	777,302		752,774
Year One writeoff	0		0
Year Two writeoff	0		0
Year Three writeoff	0		0
Year Four writeoff	0		0
Year Five writeoff	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		19
Revolving Loans Converted to Term	0		0
Total Loans	0		19
CRE - owner occupied | Pass
Financing Receivable, Recorded Investment [Line Items]
2023	24,861		58,544
2022	57,943		89,616
2021	86,027		58,798
2020	56,267		46,465
2019	45,668		80,361
Prior	262,877		192,345
Revolving Loans Amortized Cost Basis	27,624		28,023
Revolving Loans Converted to Term	172,799		158,652
Total Loans	734,066		712,804
CRE - owner occupied | Special Mention
Financing Receivable, Recorded Investment [Line Items]
2023	0		3,258
2022	3,193		2,384
2021	2,328		649
2020	2,130		639
2019	627		1,472
Prior	13,392		11,962
Revolving Loans Amortized Cost Basis	706		743
Revolving Loans Converted to Term	2,797		6,064
Total Loans	25,173		27,171
CRE - owner occupied | Substandard
Financing Receivable, Recorded Investment [Line Items]
2023	0		880
2022	874		108
2021	2,672		922
2020	895		1,480
2019	1,496		514
Prior	6,890		7,531
Revolving Loans Amortized Cost Basis	367		941
Revolving Loans Converted to Term	4,869		423
Total Loans	18,063		12,799
CRE - owner occupied | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2019	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	0		0
CRE - non-owner occupied
Financing Receivable, Recorded Investment [Line Items]
2023	46,306		144,095
2022	143,413		256,662
2021	254,782		128,538
2020	126,987		91,355
2019	86,775		62,003
Prior	377,390		325,684
Revolving Loans Amortized Cost Basis	412,488		406,465
Revolving Loans Converted to Term	301,897		296,396
Total Loans	1,750,038		1,711,198
Year One writeoff	0		0
Year Two writeoff	0		0
Year Three writeoff	0		0
Year Four writeoff	0		0
Year Five writeoff	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	0		0
CRE - non-owner occupied | Pass
Financing Receivable, Recorded Investment [Line Items]
2023	46,146		143,106
2022	142,425		255,699
2021	253,828		111,306
2020	109,985		86,560
2019	85,180		60,646
Prior	321,511		275,458
Revolving Loans Amortized Cost Basis	393,412		387,559
Revolving Loans Converted to Term	268,762		265,348
Total Loans	1,621,249		1,585,682
CRE - non-owner occupied | Special Mention
Financing Receivable, Recorded Investment [Line Items]
2023	160		42
2022	41		827
2021	819		16,109
2020	15,879		1,311
2019	1,292		109
Prior	31,513		29,648
Revolving Loans Amortized Cost Basis	18,976		18,806
Revolving Loans Converted to Term	5,781		3,506
Total Loans	74,461		70,358
CRE - non-owner occupied | Substandard
Financing Receivable, Recorded Investment [Line Items]
2023	0		947
2022	947		136
2021	135		1,123
2020	157		2,996
2019	303		1,248
Prior	24,366		20,578
Revolving Loans Amortized Cost Basis	100		100
Revolving Loans Converted to Term	27,354		27,542
Total Loans	53,362		54,670
CRE - non-owner occupied | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	0		0
2021	0		0
2020	966		488
2019	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	966		488
Commercial & industrial and other business loans
Financing Receivable, Recorded Investment [Line Items]
2023	49,791		161,971
2022	141,943		136,400
2021	130,062		96,813
2020	88,620		36,606
2019	32,866		37,590
Prior	131,386		102,684
Revolving Loans Amortized Cost Basis	393,464		364,529
Revolving Loans Converted to Term	66,449		64,037
Total Loans	1,034,581		1,000,630
Year One writeoff	0		0
Year Two writeoff	64		160
Year Three writeoff	0		0
Year Four writeoff	0		0
Year Five writeoff	0		0
Prior	0		36
Revolving Loans Amortized Cost Basis	195		569
Revolving Loans Converted to Term	2		0
Total Loans	261		765
Commercial & industrial and other business loans | Pass
Financing Receivable, Recorded Investment [Line Items]
2023	49,198		146,627
2022	127,095		133,529
2021	126,984		94,764
2020	85,922		34,572
2019	31,062		34,714
Prior	126,058		99,525
Revolving Loans Amortized Cost Basis	361,048		337,388
Revolving Loans Converted to Term	58,138		55,222
Total Loans	965,505		936,341
Commercial & industrial and other business loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
2023	579		15,306
2022	14,812		2,071
2021	2,134		1,491
2020	2,101		1,557
2019	1,412		2,553
Prior	4,416		1,854
Revolving Loans Amortized Cost Basis	16,650		16,341
Revolving Loans Converted to Term	2,250		8,045
Total Loans	44,354		49,218
Commercial & industrial and other business loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
2023	14		38
2022	36		800
2021	944		558
2020	597		477
2019	392		323
Prior	912		1,305
Revolving Loans Amortized Cost Basis	15,766		10,800
Revolving Loans Converted to Term	6,061		770
Total Loans	24,722		15,071
Commercial & industrial and other business loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	0		0
2021	0		0
2020	0		0
2019	0		0
Prior	0		0
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	0		0
Consumer mortgage
Financing Receivable, Recorded Investment [Line Items]
2023	91,635		503,463
2022	499,124		506,770
2021	497,271		624,951
2020	612,793		304,666
2019	298,796		230,933
Prior	1,150,679		956,818
Revolving Loans Amortized Cost Basis	37,999		39,997
Revolving Loans Converted to Term	129,170		117,420
Total Loans	3,317,467		3,285,018
Year One writeoff	0		0
Year Two writeoff	0		0
Year Three writeoff	0		0
Year Four writeoff			0
Year Five writeoff	0		85
Prior	93		584
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	93		669
Consumer mortgage | FICO AB
Financing Receivable, Recorded Investment [Line Items]
2023	64,407		354,967
2022	349,747		353,556
2021	347,507		457,635
2020	448,643		200,034
2019	196,357		157,438
Prior	745,983		611,778
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	93,779		86,262
Total Loans	2,246,423		2,221,670
Consumer mortgage | FICO AB | Performing
Financing Receivable, Recorded Investment [Line Items]
2023	64,407		354,967
2022	349,747		353,185
2021	347,072		456,871
2020	448,044		199,429
2019	195,758		157,159
Prior	740,946		606,591
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	93,588		86,067
Total Loans	2,239,562		2,214,269
Consumer mortgage | FICO AB | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	0		371
2021	435		764
2020	599		605
2019	599		279
Prior	5,037		5,187
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	191		195
Total Loans	6,861		7,401
Consumer mortgage | FICO CDE
Financing Receivable, Recorded Investment [Line Items]
2023	27,228		148,496
2022	149,377		153,214
2021	149,764		167,316
2020	164,150		104,632
2019	102,439		73,495
Prior	404,696		345,040
Revolving Loans Amortized Cost Basis	37,999		39,997
Revolving Loans Converted to Term	35,391		31,158
Total Loans	1,071,044		1,063,348
Consumer mortgage | FICO CDE | Performing
Financing Receivable, Recorded Investment [Line Items]
2023	27,228		148,443
2022	148,818		150,585
2021	146,921		164,839
2020	162,313		103,003
2019	100,186		71,710
Prior	390,278		331,839
Revolving Loans Amortized Cost Basis	37,639		39,630
Revolving Loans Converted to Term	34,341		30,098
Total Loans	1,047,724		1,040,147
Consumer mortgage | FICO CDE | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
2023	0		53
2022	559		2,629
2021	2,843		2,477
2020	1,837		1,629
2019	2,253		1,785
Prior	14,418		13,201
Revolving Loans Amortized Cost Basis	360		367
Revolving Loans Converted to Term	1,050		1,060
Total Loans	23,320		23,201
Consumer indirect
Financing Receivable, Recorded Investment [Line Items]
2023	163,183		681,908
2022	641,170		573,242
2021	524,351		273,136
2020	240,215		71,470
2019	60,265		45,222
Prior	86,844		58,462
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	1,716,028		1,703,440
Year One writeoff	0		926
Year Two writeoff	943		3,595
Year Three writeoff	1,282		1,969
Year Four writeoff	393		1,171
Year Five writeoff	175		570
Prior	267		1,121
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	3,060		9,352
Consumer indirect | Performing
Financing Receivable, Recorded Investment [Line Items]
2023	163,183		681,824
2022	640,930		572,799
2021	524,062		273,035
2020	240,137		71,428
2019	60,246		45,203
Prior	86,605		58,375
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	1,715,163		1,702,664
Consumer indirect | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
2023	0		84
2022	240		443
2021	289		101
2020	78		42
2019	19		19
Prior	239		87
Revolving Loans Amortized Cost Basis	0		0
Revolving Loans Converted to Term	0		0
Total Loans	865		776
Consumer direct
Financing Receivable, Recorded Investment [Line Items]
2023	22,640		80,202
2022	69,915		52,867
2021	46,532		26,664
2020	22,634		8,282
2019	6,682		4,606
Prior	8,599		5,720
Revolving Loans Amortized Cost Basis	6,747		6,887
Revolving Loans Converted to Term	1		1
Total Loans	183,750		185,229
Year One writeoff	0		206
Year Two writeoff	274		813
Year Three writeoff	254		450
Year Four writeoff	227		110
Year Five writeoff	17		110
Prior	41		159
Revolving Loans Amortized Cost Basis	60		161
Revolving Loans Converted to Term	0		0
Total Loans	873		2,009
Consumer direct | Performing
Financing Receivable, Recorded Investment [Line Items]
2023	22,640		80,169
2022	69,889		52,826
2021	46,515		26,617
2020	22,634		8,282
2019	6,682		4,604
Prior	8,574		5,697
Revolving Loans Amortized Cost Basis	6,722		6,875
Revolving Loans Converted to Term	1		1
Total Loans	183,657		185,071
Consumer direct | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
2023	0		33
2022	26		41
2021	17		47
2020	0		0
2019	0		2
Prior	25		23
Revolving Loans Amortized Cost Basis	25		12
Revolving Loans Converted to Term	0		0
Total Loans	93		158
Home equity
Financing Receivable, Recorded Investment [Line Items]
2023	11,042		61,065
2022	59,874		62,963
2021	61,356		63,112
2020	61,142		31,347
2019	30,461		25,981
Prior	67,401		45,365
Revolving Loans Amortized Cost Basis	126,773		127,992
Revolving Loans Converted to Term	28,007		28,690
Total Loans	446,056		446,515
Year One writeoff	0		0
Year Two writeoff	0		0
Year Three writeoff	23		0
Year Four writeoff	0		64
Year Five writeoff	0		0
Prior	0		44
Revolving Loans Amortized Cost Basis	0		11
Revolving Loans Converted to Term	0		0
Total Loans	23		119
Home equity | Performing
Financing Receivable, Recorded Investment [Line Items]
2023	11,042		61,065
2022	59,649		62,801
2021	61,180		63,102
2020	61,104		31,094
2019	30,104		25,721
Prior	66,718		44,832
Revolving Loans Amortized Cost Basis	125,865		126,939
Revolving Loans Converted to Term	27,626		28,177
Total Loans	443,288		443,731
Home equity | Nonperforming
Financing Receivable, Recorded Investment [Line Items]
2023	0		0
2022	225		162
2021	176		10
2020	38		253
2019	357		260
Prior	683		533
Revolving Loans Amortized Cost Basis	908		1,053
Revolving Loans Converted to Term	381		513
Total Loans	$ 2,768		$ 2,784