STOCK-BASED COMPENSATION - SUMMARY OF RESTRICTED STOCK UNITS AND RESTRICTED STOCK AWARDS AND PSUs (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
RSUs outstanding
Number of Restricted Stock Units / Awards
Beginning outstanding balance (in shares)	802
Released (in shares)	(319)
Forfeited (in shares)	(236)
Ending outstanding balance (in shares)	247	802
Weighted Average Grant Date Fair Value
Beginning outstanding balance (in dollars per share)	$ 6.98
Released (in dollars per share)	7.45
Forfeited (in dollars per share)	6.67
Ending outstanding balance (in dollars per share)	$ 6.67	$ 6.98
Weighted average remaining contractual life / recognition period, Outstanding	9 months 14 days	1 year
Aggregate intrinsic value, Outstanding	$ 1,690	$ 9,057
RSAs outstanding
Number of Restricted Stock Units / Awards
Beginning outstanding balance (in shares)	130
Granted (in shares)	0
Released (in shares)	(130)
Forfeited (in shares)	0
Ending outstanding balance (in shares)	0	130
Weighted Average Grant Date Fair Value
Beginning outstanding balance (in dollars per share)	$ 6.53
Granted (in dollars per share)	0
Released (in dollars per share)	6.53
Forfeited (in dollars per share)	0
Ending outstanding balance (in dollars per share)	$ 0	$ 6.53
Weighted average remaining contractual life / recognition period, Outstanding	0 years	5 months 12 days
PSUs outstanding
Number of Restricted Stock Units / Awards
Beginning outstanding balance (in shares)	250
Forfeited (in shares)	(160)
Ending outstanding balance (in shares)	90	250
Weighted Average Grant Date Fair Value
Beginning outstanding balance (in dollars per share)	$ 6.20
Forfeited (in dollars per share)	6.20
Ending outstanding balance (in dollars per share)	$ 6.20	$ 6.20
Weighted average remaining contractual life / recognition period, Outstanding	1 year 3 months 29 days	2 years 29 days

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Intrinsic value of outstanding award under share-based payment arrangement. Excludes share and unit options.

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