SEGMENT REPORTING - Summary of Long-lived Assets by Geographical Region (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 83,522	$ 87,086
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	50,614	52,627
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	31,390	32,924
U.K.
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 1,518	$ 1,535