OPERATING SEGMENT AND GEOGRAPHIC INFORMATION - Summary of Revenues by Country (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 70,035	$ 68,907	$ 69,642	$ 72,133	$ 74,117	$ 73,940	$ 74,315	$ 75,924	$ 280,718	$ 298,297	$ 318,293
United States
Segment Reporting Information [Line Items]
Revenues									220,018	231,943	245,853
Canada
Segment Reporting Information [Line Items]
Revenues									38,750	44,206	47,021
Other countries
Segment Reporting Information [Line Items]
Revenues									$ 21,950	$ 22,148	$ 25,419