SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair value Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair value measurements
Loss on equity securities			$ 4,724		$ 2,033
Acquired additional securities			25,362	$ 3,000	$ 18,107
Other Assets
Fair value measurements
Loss on equity securities			4,700
Acquired additional securities			1,200
Carrying Value
Fair value measurements
Long-term debt	$ 86,400		86,400	91,000
Carrying Value | Other Assets
Fair value measurements
Strategic investments	19,600		19,600	23,100
Short Term Investments
Fair value measurements
Equity investments	200		200	300
Loss on equity securities			100	300
Other investments
Fair value measurements
Equity investments	10,200		10,200
Purchase of other investments			10,000
Unrealized (loss) gain			200
Other investments | Renewable energy partnership
Fair value measurements
Equity investments	2,500		2,500
Equity in earnings of an unconsolidated affiliate	100
Investment cost		$ 14,400
Investment Tax Credit		$ 12,000
Level 2 | Estimated Fair Value
Fair value measurements
Long-term debt	86,900		86,900	91,600
Level 2 | Interest rate swap
Fair value measurements
Unrealized (loss) gain			(200)
Recurring basis
Fair value measurements
Total assets and liabilities measured at fair value	15,769		15,769	17,151
Recurring basis | Certificate of deposit
Fair value measurements
Cash and cash equivalents	380		380	380
Recurring basis | Money market funds
Fair value measurements
Cash and cash equivalents	2,329		2,329	2,266
Recurring basis | Short Term Investments
Fair value measurements
Investments	416		416	393
Recurring basis | Other investments
Fair value measurements
Investments	12,700		12,700
Recurring basis | Commercial paper
Fair value measurements
Cash and cash equivalents				13,972
Recurring basis | Interest rate swap
Fair value measurements
Derivative assets				140
Derivative liabilities	(56)		(56)
Recurring basis | Level 1
Fair value measurements
Total assets and liabilities measured at fair value	2,745		2,745	2,659
Recurring basis | Level 1 | Money market funds
Fair value measurements
Cash and cash equivalents	2,329		2,329	2,266
Recurring basis | Level 1 | Short Term Investments
Fair value measurements
Investments	416		416	393
Recurring basis | Level 2
Fair value measurements
Total assets and liabilities measured at fair value	324		324	14,492
Recurring basis | Level 2 | Certificate of deposit
Fair value measurements
Cash and cash equivalents	380		380	380
Recurring basis | Level 2 | Commercial paper
Fair value measurements
Cash and cash equivalents				13,972
Recurring basis | Level 2 | Interest rate swap
Fair value measurements
Derivative assets				$ 140
Derivative liabilities	(56)		(56)
Recurring basis | Level 3
Fair value measurements
Total assets and liabilities measured at fair value	12,700		12,700
Recurring basis | Level 3 | Other investments
Fair value measurements
Investments	$ 12,700		$ 12,700