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		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Allowance for credit losses
Beginning Balance	$ 47,576
Charge-offs	(4,455)
Recoveries	3,451
Provision for credit losses	56,646	$ 7,654
Ending balance	139,073	46,321	$ 47,576
Allowance for loan and lease losses
Beginning balance	47,576	39,539	39,539
Charge-offs		(1,550)
Recoveries		678
Provision (credit)		7,495
Provision (credit) for acquired loans		159
Ending balance	139,073	46,321	47,576
Loans evaluated on an individual basis				$ 20		$ 5,654
Loans evaluated on a collective basis				139,053		40,440
Acquired loans individually evaluated for impairment						227
Allowance for credit losses	139,073	46,321	47,576	139,073	$ 47,576	46,321
Loans individually evaluated for impairment				26,895		47,032
Loans evaluated on a collective basis				8,508,091		4,670,155
Acquired nonimpaired loans					3,200,000	3,933,340
Acquired impaired loans		42,490	26,935
Ending balance	8,534,986	8,693,017	8,464,562
Reverse mortgage, fair value				15,800	16,600	16,200
Loans purchased with credit deterioration	100
Performing TDRs	14,070	15,000	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	22,849
Charge-offs	(3,064)
Recoveries	2,847
Provision for credit losses	23,392
Ending balance	65,771	21,016	22,849
Allowance for loan and lease losses
Beginning balance		14,211	14,211
Charge-offs		(742)
Recoveries		358
Provision (credit)		7,123
Provision (credit) for acquired loans		66
Ending balance		21,016
Loans evaluated on an individual basis				20		4,588
Loans evaluated on a collective basis				65,751		16,427
Acquired loans individually evaluated for impairment						1
Allowance for credit losses	65,771	21,016	22,849	65,771	22,849	21,016
Loans individually evaluated for impairment				8,843		16,109
Loans evaluated on a collective basis				2,289,529		1,415,689
Acquired nonimpaired loans						782,160
Acquired impaired loans		6,130	1,564
Ending balance	2,298,372	2,220,088	2,235,428
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	4,616
Charge-offs	(283)
Recoveries	125
Provision for credit losses	6,555
Ending balance	9,541	4,949	4,616
Allowance for loan and lease losses
Beginning balance		5,057	5,057
Charge-offs		0
Recoveries		3
Provision (credit)		(111)
Provision (credit) for acquired loans		0
Ending balance		4,949
Loans evaluated on an individual basis				0		0
Loans evaluated on a collective basis				9,541		4,856
Acquired loans individually evaluated for impairment						93
Allowance for credit losses	9,541	4,949	4,616	9,541	4,616	4,949
Loans individually evaluated for impairment				4,818		5,384
Loans evaluated on a collective basis				1,308,375		1,198,337
Acquired nonimpaired loans						105,154
Acquired impaired loans		4,070	6,757
Ending balance	1,313,193	1,312,945	1,296,466
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	7,452
Charge-offs	(51)
Recoveries	29
Provision for credit losses	17,508
Ending balance	26,600	6,679	7,452
Allowance for loan and lease losses
Beginning balance		6,806	6,806
Charge-offs		(2)
Recoveries		29
Provision (credit)		(156)
Provision (credit) for acquired loans		2
Ending balance		6,679
Loans evaluated on an individual basis				0		0
Loans evaluated on a collective basis				26,600		6,600
Acquired loans individually evaluated for impairment						79
Allowance for credit losses	26,600	6,679	7,452	26,600	7,452	6,679
Loans individually evaluated for impairment				4,691		3,999
Loans evaluated on a collective basis				2,218,426		753,911
Acquired nonimpaired loans						1,575,527
Acquired impaired loans		19,715	8,670
Ending balance	2,223,117	2,353,152	2,222,976
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	3,891
Charge-offs	0
Recoveries	5
Provision for credit losses	621
Ending balance	5,198	4,044	3,891
Allowance for loan and lease losses
Beginning balance		3,712	3,712
Charge-offs		0
Recoveries		1
Provision (credit)		331
Provision (credit) for acquired loans		0
Ending balance		4,044
Loans evaluated on an individual basis				0		367
Loans evaluated on a collective basis				5,198		3,663
Acquired loans individually evaluated for impairment						14
Allowance for credit losses	5,198	4,044	3,891	5,198	3,891	4,044
Loans individually evaluated for impairment				96		2,781
Loans evaluated on a collective basis				626,157		347,035
Acquired nonimpaired loans						225,245
Acquired impaired loans		636	491
Ending balance	626,253	575,697	581,082
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	1,381
Charge-offs	(143)
Recoveries	91
Provision for credit losses	2,742
Ending balance	11,593	1,401	1,381
Allowance for loan and lease losses
Beginning balance		1,428	1,428
Charge-offs		(122)
Recoveries		(14)
Provision (credit)		51
Provision (credit) for acquired loans		58
Ending balance		1,401
Loans evaluated on an individual basis				0		533
Loans evaluated on a collective basis				11,593		830
Acquired loans individually evaluated for impairment						38
Allowance for credit losses	11,593	1,401	1,381	11,593	1,381	1,401
Loans individually evaluated for impairment				6,152		10,590
Loans evaluated on a collective basis				949,613		130,499
Acquired nonimpaired loans						949,804
Acquired impaired loans		8,483	7,326
Ending balance	955,765	1,099,376	999,879
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	7,387
Charge-offs	(914)
Recoveries	354
Provision for credit losses	5,828
Ending balance	20,370	8,232	7,387
Allowance for loan and lease losses
Beginning balance		8,325	8,325
Charge-offs		(684)
Recoveries		301
Provision (credit)		257
Provision (credit) for acquired loans		33
Ending balance		8,232
Loans evaluated on an individual basis				0		166
Loans evaluated on a collective basis				20,370		8,064
Acquired loans individually evaluated for impairment						2
Allowance for credit losses	20,370	8,232	7,387	20,370	7,387	8,232
Loans individually evaluated for impairment				2,295		8,169
Loans evaluated on a collective basis				$ 1,115,991		824,684
Acquired nonimpaired loans						$ 295,450
Acquired impaired loans		3,456	2,127
Ending balance	1,118,286	$ 1,131,759	1,128,731
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