SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020		Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan loss	$ 27,700	$ 169,500
Accumulated deficit	$ 9,720,270	$ 10,667,388	[1]
Cumulative effect of change in accounting principle for credit losses
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] (1)Derived from the audited consolidated financial statements at December 31, 2020.