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LOANS AND ALLOWANCE FOR CREDIT LOSSES - Schedule of Composition of Loans and Impact of Adoption (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2024
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 51,148,063		$ 51,148,063		$ 48,764,162
Allowance for credit losses on loans	(579,658)	$ (572,178)	(579,658)	$ (572,178)	(569,520)	$ (580,511)	$ (565,109)	$ (392,522)
Net loans	50,568,405		50,568,405		48,194,642
Taxable	730,196	731,633	2,138,505	1,905,907
Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	51,148,063		51,148,063		48,764,162
Taxable			422,500		540,100
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	16,260,724		16,260,724		14,983,861
Allowance for credit losses on loans	(253,214)	(234,983)	(253,214)	(234,983)	(244,670)	(253,854)	(215,917)	(148,722)
Commercial \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0		0		(220,410)
Commercial \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	16,260,724		16,260,724		14,763,451
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	22,692,502		22,692,502		22,050,007
Allowance for credit losses on loans	(241,056)	(282,827)	(241,056)	(282,827)	(268,332)	(263,706)	(294,303)	(200,309)
Commercial real estate \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	0		0		(175,670)
Commercial real estate \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	22,692,502		22,692,502		21,874,337
BBCC
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses on loans	0	(2,458)	0	(2,458)	(2,371)	(2,961)	(2,488)	(2,813)
BBCC \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans					396,080
BBCC \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans					396,080
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	8,770,515		8,770,515		8,467,496
Allowance for credit losses on loans	(54,603)	(31,431)	(54,603)	(31,431)	(34,394)	(40,450)	(31,850)	(22,922)
Residential real estate \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	165,020		165,020		0
Residential real estate \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	8,935,535		8,935,535		8,467,496
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	3,424,322		3,424,322		3,262,798
Consumer \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	(3,424,322)		(3,424,322)		(3,262,798)
Indirect
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses on loans	(7,313)	(7,961)	(7,313)	(7,961)	(8,021)	(7,228)	(8,430)	(8,434)
Indirect \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,288,306		1,288,306		1,075,235
Indirect \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,288,306		1,288,306		1,075,235
Direct
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses on loans	0	(2,418)	0	(2,418)	(2,478)	(2,273)	(2,564)	(2,304)
Direct \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans					649,297
Direct \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans					649,297
Home equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses on loans	(19,844)	$ (10,100)	(19,844)	$ (10,100)	(9,254)	(10,039)	$ (9,557)	$ (7,018)
Home equity \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,620,884		1,620,884		1,538,266
Home equity \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,620,884		1,620,884		1,538,266
Other consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses on loans	(3,628)		(3,628)		0	$ 0
Other consumer \| Portfolio Segment Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	350,112		350,112
Other consumer \| Portfolio Segment After Reclassifications
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	350,112		350,112
Direct Finance Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 51,000		$ 51,000		$ 75,100