REVENUE - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of revenue
Revenue	$ 66,807	$ 56,862	$ 71,466	$ 50,652	$ 44,355	$ 53,467	$ 47,975	$ 43,121	$ 245,787	$ 188,918
Canada
Disaggregation of revenue
Revenue									16,967
United States
Disaggregation of revenue
Revenue									228,820	188,918
Cost-plus reimbursement contracts
Disaggregation of revenue
Revenue									210,538	158,278
Fixed-price contracts
Disaggregation of revenue
Revenue									$ 35,249	$ 30,640