SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2021	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan loss		$ 33,400	$ 169,500
Accumulated deficit		$ 9,251,804	10,667,388	[1]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan loss			37,400
Off-balance sheet credit loss, liability			2,200
Accumulated deficit			$ 39,600

[1] Derived from the audited consolidated financial statements at December 31, 2020.