ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Allowance for Loan Losses and Loan Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Allowance for credit losses
Beginning Balance	$ 232,192		$ 47,576
Charge-offs	(2,632)		(9,911)
Recoveries	450		5,090
Provision for credit losses	2,716	$ 4,121	154,116	$ 23,970
Ending balance	232,726	47,671	232,726	47,671	$ 47,576
Allowance for loan and lease losses
Beginning balance		45,364		39,539	39,539
Charge-offs		(2,496)		(18,374)
Recoveries		682		2,536
Provision (credit)		3,980		23,126
Provision (credit) for acquired loans		141		844
Ending balance		47,671		47,671
Loans evaluated on an individual basis						$ 16		$ 4,348
Loans evaluated on a collective basis						232,710		43,148
Acquired loans individually evaluated for impairment								175
Allowance for credit losses	232,192	47,671	47,576	47,671	47,576	232,726	$ 47,576	47,671
Loans individually evaluated for impairment						32,849		52,256
Loans evaluated on a collective basis						9,289,710		5,013,755
Acquired nonimpaired loans							3,200,000	3,421,089
Acquired impaired loans				31,815
Ending balance						9,322,559		8,518,915
Reverse mortgage, fair value						12,500	16,600	17,700
Loans purchased with credit deterioration			100
Performing TDRs			15,670	14,100	14,281
Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance			35,855
Ending balance					35,855
Allowance for loan and lease losses
Allowance for credit losses			35,855		35,855		35,855
Commercial
Allowance for loan and lease losses
Acquired nonimpaired loans							2,200,000
Commercial | Commercial and industrial
Allowance for credit losses
Beginning Balance	144,225		22,849
Charge-offs	(2,254)		(7,390)
Recoveries	223		4,038
Provision for credit losses	6,335		109,285
Ending balance	148,529	24,504	148,529	24,504	22,849
Allowance for loan and lease losses
Beginning balance		22,004		14,211	14,211
Charge-offs		(1,441)		(15,185)
Recoveries		297		858
Provision (credit)		3,595		24,286
Provision (credit) for acquired loans		49		334
Ending balance		24,504		24,504
Loans evaluated on an individual basis						16		3,534
Loans evaluated on a collective basis						148,513		20,969
Acquired loans individually evaluated for impairment								1
Allowance for credit losses	148,529	24,504	148,529	24,504	22,849	148,529	22,849	24,504
Loans individually evaluated for impairment						14,946		21,135
Loans evaluated on a collective basis						3,115,408		1,657,748
Acquired nonimpaired loans								605,804
Acquired impaired loans				1,664
Ending balance						3,130,354		2,286,351
Commercial | Commercial and industrial | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance			19,747
Ending balance					19,747
Allowance for loan and lease losses
Allowance for credit losses			19,747		19,747		19,747
Commercial | Owner-occupied commercial
Allowance for credit losses
Beginning Balance	8,956		4,616
Charge-offs	0		(336)
Recoveries	8		133
Provision for credit losses	1,420		7,443
Ending balance	10,384	4,504	10,384	4,504	4,616
Allowance for loan and lease losses
Beginning balance		4,480		5,057	5,057
Charge-offs		(12)		(20)
Recoveries		4		85
Provision (credit)		23		(614)
Provision (credit) for acquired loans		9		(4)
Ending balance		4,504		4,504
Loans evaluated on an individual basis						0		156
Loans evaluated on a collective basis						10,384		4,269
Acquired loans individually evaluated for impairment								79
Allowance for credit losses	10,384	4,504	10,384	4,504	4,616	10,384	4,616	4,504
Loans individually evaluated for impairment						5,132		9,263
Loans evaluated on a collective basis						1,339,362		935,306
Acquired nonimpaired loans								322,030
Acquired impaired loans				6,809
Ending balance						1,344,494		1,273,408
Commercial | Owner-occupied commercial | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance			(1,472)
Ending balance					(1,472)
Allowance for loan and lease losses
Allowance for credit losses			(1,472)		(1,472)		(1,472)
Commercial | Commercial mortgages
Allowance for credit losses
Beginning Balance	38,397		7,452
Charge-offs	(4)		(55)
Recoveries	6		38
Provision for credit losses	(4,140)		25,162
Ending balance	34,259	6,737	34,259	6,737	7,452
Allowance for loan and lease losses
Beginning balance		6,544		6,806	6,806
Charge-offs		2		(153)
Recoveries		120		547
Provision (credit)		97		(533)
Provision (credit) for acquired loans		(26)		70
Ending balance		6,737		6,737
Loans evaluated on an individual basis						0		0
Loans evaluated on a collective basis						34,259		6,690
Acquired loans individually evaluated for impairment								47
Allowance for credit losses	34,259	6,737	34,259	6,737	7,452	34,259	7,452	6,737
Loans individually evaluated for impairment						4,878		2,325
Loans evaluated on a collective basis						2,162,630		1,019,723
Acquired nonimpaired loans								1,237,282
Acquired impaired loans				12,655
Ending balance						2,167,508		2,271,985
Commercial | Commercial mortgages | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance			1,662
Ending balance					1,662
Allowance for loan and lease losses
Allowance for credit losses			1,662		1,662		1,662
Commercial | Construction
Allowance for credit losses
Beginning Balance	10,126		3,891
Charge-offs	0		0
Recoveries	0		5
Provision for credit losses	104		5,653
Ending balance	10,230	2,881	10,230	2,881	3,891
Allowance for loan and lease losses
Beginning balance		2,984		3,712	3,712
Charge-offs		0		(42)
Recoveries		2		4
Provision (credit)		(105)		(787)
Provision (credit) for acquired loans		0		(6)
Ending balance		2,881		2,881
Loans evaluated on an individual basis						0		0
Loans evaluated on a collective basis						10,230		2,873
Acquired loans individually evaluated for impairment								8
Allowance for credit losses	10,230	2,881	3,891	2,881	3,891	10,230	3,891	2,881
Loans individually evaluated for impairment						83		0
Loans evaluated on a collective basis						666,234		375,664
Acquired nonimpaired loans								138,251
Acquired impaired loans				487
Ending balance						666,317		514,402
Commercial | Construction | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance			681
Ending balance					681
Allowance for loan and lease losses
Allowance for credit losses			681		681		681
Residential
Allowance for credit losses
Beginning Balance	9,171		1,381
Charge-offs	0		(175)
Recoveries	26		141
Provision for credit losses	(743)		(415)
Ending balance	8,454	1,330	8,454	1,330	1,381
Allowance for loan and lease losses
Beginning balance		1,358		1,428	1,428
Charge-offs		(3)		(288)
Recoveries		(60)		(76)
Provision (credit)		43		118
Provision (credit) for acquired loans		(8)		148
Ending balance		1,330		1,330
Loans evaluated on an individual basis						0		478
Loans evaluated on a collective basis						8,454		813
Acquired loans individually evaluated for impairment								39
Allowance for credit losses	8,454	1,330	8,454	1,330	1,381	8,454	1,381	1,330
Loans individually evaluated for impairment						5,307		12,031
Loans evaluated on a collective basis						839,688		138,121
Acquired nonimpaired loans								876,089
Acquired impaired loans				7,655
Ending balance						844,995		1,033,896
Residential | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance			7,522
Ending balance					7,522
Allowance for loan and lease losses
Allowance for credit losses			7,522		7,522		7,522
Consumer
Allowance for credit losses
Beginning Balance	21,317		7,387
Charge-offs	(374)		(1,955)
Recoveries	187		735
Provision for credit losses	(260)		6,988
Ending balance	20,870	7,715	20,870	7,715	7,387
Allowance for loan and lease losses
Beginning balance		7,994		8,325	8,325
Charge-offs		(1,042)		(2,686)
Recoveries		319		1,118
Provision (credit)		327		656
Provision (credit) for acquired loans		117		302
Ending balance		7,715		7,715
Loans evaluated on an individual basis						0		180
Loans evaluated on a collective basis						20,870		7,534
Acquired loans individually evaluated for impairment								1
Allowance for credit losses	$ 20,870	$ 7,715	20,870	7,715	7,387	20,870	7,387	7,715
Loans individually evaluated for impairment						2,503		7,502
Loans evaluated on a collective basis						1,166,388		887,193
Acquired nonimpaired loans								241,633
Acquired impaired loans				$ 2,545
Ending balance						$ 1,168,891		$ 1,138,873
Consumer | Impact of adoption ASC 326
Allowance for credit losses
Beginning Balance			7,715
Ending balance					7,715
Allowance for loan and lease losses
Allowance for credit losses			$ 7,715		$ 7,715		$ 7,715