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		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Allowance for credit losses
Beginning Balance	$ 139,073		$ 47,576
Charge-offs	(2,824)		(7,279)
Recoveries	1,189		4,640
Provision for credit losses	94,754	$ 12,195	151,400	$ 19,849
Ending balance	232,192	45,364	232,192	45,364	$ 47,576
Allowance for loan and lease losses
Beginning balance		46,321		39,539	39,539
Charge-offs		(14,328)		(15,878)
Recoveries		1,176		1,854
Provision (credit)		11,651		19,146
Provision (credit) for acquired loans		544		703
Ending balance		45,364		45,364
Loans evaluated on an individual basis						$ 18		$ 4,995
Loans evaluated on a collective basis						232,174		40,190
Acquired loans individually evaluated for impairment								179
Allowance for credit losses	232,192	45,364	232,192	45,364	47,576	232,192	$ 47,576	45,364
Loans individually evaluated for impairment						33,533		51,136
Loans evaluated on a collective basis						9,302,836		4,816,880
Acquired nonimpaired loans							3,200,000	3,699,730
Acquired impaired loans				33,932
Ending balance						9,336,369		8,601,678
Reverse mortgage, fair value						16,100	16,600	15,900
Loans purchased with credit deterioration			100
Performing TDRs			14,550	14,200	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	65,771		22,849
Charge-offs	(2,072)		(5,136)
Recoveries	968		3,815
Provision for credit losses	79,558		102,950
Ending balance	144,225	22,004	144,225	22,004	22,849
Allowance for loan and lease losses
Beginning balance		21,016		14,211	14,211
Charge-offs		(13,002)		(13,744)
Recoveries		203		561
Provision (credit)		13,568		20,691
Provision (credit) for acquired loans		219		285
Ending balance		22,004		22,004
Loans evaluated on an individual basis						18		4,324
Loans evaluated on a collective basis						144,207		17,679
Acquired loans individually evaluated for impairment								1
Allowance for credit losses	144,225	22,004	144,225	22,004	22,849	144,225	22,849	22,004
Loans individually evaluated for impairment						15,634		21,171
Loans evaluated on a collective basis						3,151,200		1,575,810
Acquired nonimpaired loans								738,579
Acquired impaired loans				4,964
Ending balance						3,166,834		2,340,524
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	9,541		4,616
Charge-offs	(53)		(336)
Recoveries	0		125
Provision for credit losses	(532)		6,023
Ending balance	8,956	4,480	8,956	4,480	4,616
Allowance for loan and lease losses
Beginning balance		4,949		5,057	5,057
Charge-offs		(8)		(8)
Recoveries		78		81
Provision (credit)		(526)		(637)
Provision (credit) for acquired loans		(13)		(13)
Ending balance		4,480		4,480
Loans evaluated on an individual basis						0		0
Loans evaluated on a collective basis						8,956		4,401
Acquired loans individually evaluated for impairment								79
Allowance for credit losses	8,956	4,480	4,616	4,480	4,616	8,956	4,616	4,480
Loans individually evaluated for impairment						5,425		8,753
Loans evaluated on a collective basis						1,331,828		1,168,864
Acquired nonimpaired loans								99,326
Acquired impaired loans				3,951
Ending balance						1,337,253		1,280,894
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	26,600		7,452
Charge-offs	0		(51)
Recoveries	3		32
Provision for credit losses	11,794		29,302
Ending balance	38,397	6,544	38,397	6,544	7,452
Allowance for loan and lease losses
Beginning balance		6,679		6,806	6,806
Charge-offs		(153)		(155)
Recoveries		398		427
Provision (credit)		(474)		(630)
Provision (credit) for acquired loans		94		96
Ending balance		6,544		6,544
Loans evaluated on an individual basis						0		0
Loans evaluated on a collective basis						38,397		6,496
Acquired loans individually evaluated for impairment								48
Allowance for credit losses	38,397	6,544	7,452	6,544	7,452	38,397	7,452	6,544
Loans individually evaluated for impairment						4,470		2,431
Loans evaluated on a collective basis						2,161,077		765,268
Acquired nonimpaired loans								1,464,739
Acquired impaired loans				13,609
Ending balance						2,165,547		2,246,047
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	5,198		3,891
Charge-offs	0		0
Recoveries	0		5
Provision for credit losses	4,928		5,549
Ending balance	10,126	2,984	10,126	2,984	3,891
Allowance for loan and lease losses
Beginning balance		4,044		3,712	3,712
Charge-offs		(42)		(42)
Recoveries		1		2
Provision (credit)		(1,013)		(682)
Provision (credit) for acquired loans		(6)		(6)
Ending balance		2,984		2,984
Loans evaluated on an individual basis						0		0
Loans evaluated on a collective basis						10,126		2,976
Acquired loans individually evaluated for impairment								8
Allowance for credit losses	10,126	2,984	10,126	2,984	3,891	10,126	3,891	2,984
Loans individually evaluated for impairment						88		0
Loans evaluated on a collective basis						638,416		324,307
Acquired nonimpaired loans								216,843
Acquired impaired loans				546
Ending balance						638,504		541,696
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	11,593		1,381
Charge-offs	(32)		(175)
Recoveries	24		115
Provision for credit losses	(2,414)		328
Ending balance	9,171	1,358	9,171	1,358	1,381
Allowance for loan and lease losses
Beginning balance		1,401		1,428	1,428
Charge-offs		(163)		(285)
Recoveries		(2)		(16)
Provision (credit)		24		75
Provision (credit) for acquired loans		98		156
Ending balance		1,358		1,358
Loans evaluated on an individual basis						0		488
Loans evaluated on a collective basis						9,171		828
Acquired loans individually evaluated for impairment								42
Allowance for credit losses	9,171	1,358	9,171	1,358	1,381	9,171	1,381	1,358
Loans individually evaluated for impairment						5,452		11,398
Loans evaluated on a collective basis						889,408		134,235
Acquired nonimpaired loans								912,288
Acquired impaired loans				7,863
Ending balance						894,860		1,065,784
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	20,370		7,387
Charge-offs	(667)		(1,581)
Recoveries	194		548
Provision for credit losses	1,420		7,248
Ending balance	21,317	7,994	21,317	7,994	7,387
Allowance for loan and lease losses
Beginning balance		8,232		8,325	8,325
Charge-offs		(960)		(1,644)
Recoveries		498		799
Provision (credit)		72		329
Provision (credit) for acquired loans		152		185
Ending balance		7,994		7,994
Loans evaluated on an individual basis						0		183
Loans evaluated on a collective basis						21,317		7,810
Acquired loans individually evaluated for impairment								1
Allowance for credit losses	$ 21,317	$ 7,994	21,317	7,994	7,387	21,317	7,387	7,994
Loans individually evaluated for impairment						2,464		7,383
Loans evaluated on a collective basis						1,130,907		848,396
Acquired nonimpaired loans								267,955
Acquired impaired loans				$ 2,999
Ending balance						$ 1,133,371		$ 1,126,733
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