FAIR VALUE MEASUREMENTS - Assets Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses)	$ 0	$ (6,305,000)	$ (36,609,000)
Total gains (losses)	0	(1,177,000)	$ 0
Nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total gains (losses)	0	(6,305,000)
Total gains (losses)	0	(1,177,000)
Balance | Nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property and equipment, net	0	8,700,000
Goodwill	0	9,425,000
Estimated fair value measurements | Nonrecurring basis | Quoted prices in active market (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property and equipment, net	0	0
Goodwill	0	0
Estimated fair value measurements | Nonrecurring basis | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property and equipment, net	0	8,700,000
Goodwill	0	0
Estimated fair value measurements | Nonrecurring basis | Significant other unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property and equipment, net	0	0
Goodwill	$ 0	$ 9,425,000