ALLOWANCE FOR CREDIT LOSSES - Changes in the Allowance for Credit Losses (Details) - USD ($) $ in Thousands				3 Months Ended			9 Months Ended
		Jan. 01, 2020		Sep. 30, 2020		Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		$ 57,650	[1]	$ 158,661		$ 61,549	$ 57,650	[1]	$ 56,542
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		61,505					61,505
Provision for Credit Losses	[2]			15,299		5,228	57,038		25,969
Loans charged off				(6,820)		(11,659)	(12,805)		(29,406)
Recoveries				1,404		1,434	5,156		3,447
Total net charge-offs				(5,416)		(10,225)	(7,649)		(25,959)
Balance at end of year		119,155		168,544	[1]	56,552	168,544	[1]	56,552
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										$ 2,175
Ending allowance on loans collectively evaluated for impairment										55,475
Impaired Financing Receivable, Related Allowance										57,650
Loans and Leases:
Ending balance of loans individually evaluated for impairment										59,600
Ending balance of loans collectively evaluated for impairment										9,142,065
Loans and Leases Receivable, Net of Deferred Income				10,200,809			10,200,809			9,201,665
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		18,584		50,421		24,586	18,584		18,746
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		9,901					9,901
Provision for Credit Losses				1,297		2,654	23,331		22,164
Loans charged off				(1,467)		(9,556)	(3,840)		(23,757)
Recoveries				265		556	2,540		1,087
Total net charge-offs				(1,202)		(9,000)	(1,300)		(22,670)
Balance at end of year		28,485		50,516		18,240	50,516		18,240
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										2,044
Ending allowance on loans collectively evaluated for impairment										16,540
Impaired Financing Receivable, Related Allowance										18,584
Loans and Leases:
Ending balance of loans individually evaluated for impairment										27,480
Ending balance of loans collectively evaluated for impairment										2,438,397
Loans and Leases Receivable, Net of Deferred Income										2,465,877
Lease financing
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		971		1,431		1,393	971		1,130
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		118					118
Provision for Credit Losses				702		(388)	1,044		(25)
Loans charged off				852		0	(852)		(100)
Recoveries				6		0	6		0
Total net charge-offs				(846)		0	(846)		(100)
Balance at end of year		1,089		1,287		1,005	1,287		1,005
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										0
Ending allowance on loans collectively evaluated for impairment										971
Impaired Financing Receivable, Related Allowance										971
Loans and Leases:
Ending balance of loans individually evaluated for impairment										223
Ending balance of loans collectively evaluated for impairment										88,141
Loans and Leases Receivable, Net of Deferred Income										88,364
Construction real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		2,381		15,357		2,919	2,381		3,413
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		11,579					11,579
Provision for Credit Losses				3,613		(152)	4,996		(714)
Loans charged off				0		0	0		0
Recoveries				0		0	14		68
Total net charge-offs				0		0	14		68
Balance at end of year		13,960		18,970		2,767	18,970		2,767
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										0
Ending allowance on loans collectively evaluated for impairment										2,381
Impaired Financing Receivable, Related Allowance										2,381
Loans and Leases:
Ending balance of loans individually evaluated for impairment										0
Ending balance of loans collectively evaluated for impairment										493,182
Loans and Leases Receivable, Net of Deferred Income										493,182
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		23,579		62,340		20,357	23,579		21,048
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		24,118					24,118
Provision for Credit Losses				12,896		1,684	28,922		1,966
Loans charged off				(3,789)		(535)	(5,830)		(1,835)
Recoveries				760		347	1,418		674
Total net charge-offs				(3,029)		(188)	(4,412)		(1,161)
Balance at end of year		47,697		72,207		21,853	72,207		21,853
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										113
Ending allowance on loans collectively evaluated for impairment										23,466
Impaired Financing Receivable, Related Allowance										23,579
Loans and Leases:
Ending balance of loans individually evaluated for impairment										10,831
Ending balance of loans collectively evaluated for impairment										4,183,820
Loans and Leases Receivable, Net of Deferred Income										4,194,651
Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		5,299		10,581		5,008	5,299		4,964
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		5,490					5,490
Provision for Credit Losses				(1,364)		702	(1,454)		841
Loans charged off				(22)		(278)	(285)		(510)
Recoveries				91		64	236		201
Total net charge-offs				69		(214)	(49)		(309)
Balance at end of year		10,789		9,286		5,496	9,286		5,496
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										18
Ending allowance on loans collectively evaluated for impairment										5,281
Impaired Financing Receivable, Related Allowance										5,299
Loans and Leases:
Ending balance of loans individually evaluated for impairment										15,162
Ending balance of loans collectively evaluated for impairment										1,040,787
Loans and Leases Receivable, Net of Deferred Income										1,055,949
Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		4,787		14,236		5,307	4,787		5,348
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		8,430					8,430
Provision for Credit Losses				(1,455)		68	(236)		427
Loans charged off				(460)		(627)	(1,155)		(1,784)
Recoveries				209		335	704		1,092
Total net charge-offs				(251)		(292)	(451)		(692)
Balance at end of year		13,217		12,530		5,083	12,530		5,083
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										0
Ending allowance on loans collectively evaluated for impairment										4,787
Impaired Financing Receivable, Related Allowance										4,787
Loans and Leases:
Ending balance of loans individually evaluated for impairment										5,700
Ending balance of loans collectively evaluated for impairment										766,169
Loans and Leases Receivable, Net of Deferred Income										771,869
Installment
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										0
Ending allowance on loans collectively evaluated for impairment										392
Impaired Financing Receivable, Related Allowance										392
Loans and Leases:
Ending balance of loans individually evaluated for impairment										204
Ending balance of loans collectively evaluated for impairment										82,385
Loans and Leases Receivable, Net of Deferred Income										82,589
Installment
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		392		1,226		395	392		362
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		801					801
Provision for Credit Losses				35		(2)	78		49
Loans charged off				(59)		(65)	(127)		(192)
Recoveries				35		93	93		202
Total net charge-offs				(24)		28	(34)		10
Balance at end of year		1,193		1,237		421	1,237		421
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										0
Loans and Leases:
Loans and Leases Receivable, Net of Deferred Income										1,959,591
Credit card
Financing Receivable, Allowance for Credit Loss [Line Items]
Balance at beginning of year		1,657		3,069		1,584	1,657		1,531
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		1,068					1,068
Provision for Credit Losses				(425)		662	357		1,261
Loans charged off				(171)		(598)	(716)		(1,228)
Recoveries				38		39	145		123
Total net charge-offs				133		559	571		1,105
Balance at end of year		$ 2,725		$ 2,511		$ 1,687	$ 2,511		$ 1,687
Allowance for credit losses:
Ending allowance on loans individually evaluated for impairment										0
Ending allowance on loans collectively evaluated for impairment										1,657
Impaired Financing Receivable, Related Allowance										1,657
Loans and Leases:
Ending balance of loans individually evaluated for impairment										0
Ending balance of loans collectively evaluated for impairment										49,184
Loans and Leases Receivable, Net of Deferred Income										$ 49,184

[1] Beginning January 1, 2020, calculation is based on current expected loss methodology. Prior to January 1, 2020, calculation was based on incurred loss methodology.

[2] Beginning January 1, 2020, calculation is based on current expected loss methodology. Prior to January 1, 2020, calculation was based on incurred loss methodology.