BASIS OF PRESENTATION AND ACCOUNTING POLICIES - Narrative (Details)		9 Months Ended	12 Months Ended
	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) branch	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($)	Mar. 26, 2020	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of branches operated | branch		170
Cash reserve required rate						0.00%
Reserve requirement		$ 0	$ 109,700,000
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Allowance for loan losses		$ 231,924,000	$ 38,189,000	$ 28,819,000	$ 208,793,000		$ 35,530,000	$ 31,793,000
Retained earnings (accumulated deficit)		587,657,000	507,950,000
Deferred interest on loans, allowance for credit losses		1,100,000
Loan Receivables
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable		76,000,000.0
Available-for-sale Debt Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable		$ 4,500,000
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan losses	$ 78,700,000		$ 78,661,000
Allowance for unfunded commitments	12,700,000
Retained earnings (accumulated deficit)	(56,700,000)
Deferred tax assets	19,000,000.0
Discounts on PCD loans reclassified to allowance for credit loss	$ 15,600,000