FAIR VALUE MEASUREMENTS - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 0	$ 2,309,000
Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		2,309,000
Recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		2,309,000
Recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0
Asset retirement obligations | Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset retirement obligations	2,322,000	2,090,000
Asset retirement obligations | Recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset retirement obligations	0	0
Asset retirement obligations | Recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset retirement obligations	0	0
Asset retirement obligations | Recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset retirement obligations	$ 2,322,000	$ 2,090,000