BASIS OF PRESENTATION AND ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	Jan. 01, 2020 USD ($)	Mar. 31, 2020 USD ($) branch	Dec. 31, 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
Allowance for loan losses	$ 78,661	149,524	38,189	$ 28,659	$ 28,819
Retained earnings (accumulated deficit)		460,153	$ 507,950
Available-for-sale debt securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable		5,300
Loan receivables
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest receivable		$ 43,600
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan losses	78,700
Allowance for unfunded commitments	12,700
Retained earnings (accumulated deficit)	(56,700)
Deferred tax assets	19,000
Discounts on PCD loans reclassified to allowance for credit loss	$ 15,600