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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Debt (Details) $ in Thousands	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Carrying Value
Fair value disclosure
Financial instruments	$ 258,807	$ 305,623
Fair Value
Fair value disclosure
Financial instruments	264,340	312,749
Credit Facility | Carrying Value | Level 2
Fair value disclosure
Financial instruments	50,000	70,000
Credit Facility | Fair Value | Level 2
Fair value disclosure
Financial instruments	50,000	70,000
Notes payable | Carrying Value | Level 2
Fair value disclosure
Financial instruments	187,715	214,216
Notes payable | Fair Value | Level 2
Fair value disclosure
Financial instruments	$ 190,519	$ 217,226
New England Pension Fund withdrawal liability | Discount Rate
Fair value disclosure
Measurement input	0.031	0.026
New England Pension Fund withdrawal liability | Carrying Value | Level 2
Fair value disclosure
Financial instruments	$ 21,092	$ 21,407
New England Pension Fund withdrawal liability | Fair Value | Level 2
Fair value disclosure
Financial instruments	$ 23,821	$ 25,523

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