BASIS OF PRESENTATION AND ACCOUNTING POLICIES - Narrative (Details)	Jan. 01, 2020 USD ($)	Jun. 30, 2020 USD ($) branch	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of branches operated | branch		170
Reserve requirement		$ 0		$ 109,700,000
Allowance for loan losses		208,793,000	$ 149,524,000	38,189,000	$ 31,793,000	$ 28,659,000	$ 28,819,000
Retained earnings (accumulated deficit)		481,948,000		$ 507,950,000
Loan receivables
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable		67,700,000
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	$ 78,661,000
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