DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Geographical (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Number of segments | segment									1
Revenue recognition	$ 57,252	$ 52,979	$ 48,470	$ 49,774	$ 47,966	$ 51,337	$ 54,357	$ 52,887	$ 208,475	$ 206,547	$ 201,576
Property and equipment, net	41,269				40,551				41,269	40,551
United States [Member]
Revenue recognition									202,881	199,663	196,886
Property and equipment, net	26,960				26,708				26,960	26,708
Canada [Member]
Revenue recognition									5,594	6,884	$ 4,690
Property and equipment, net	$ 14,309				$ 13,843				$ 14,309	$ 13,843