REVENUE - Transaction price allocated to the remaining performance obligations (Details) - Fixed-price contracts $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Transaction price allocated to the remaining performance obligations
Remaining performance obligation	$ 501,604
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Transaction price allocated to the remaining performance obligations
Remaining performance obligation	$ 173,346
Expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Transaction price allocated to the remaining performance obligations
Remaining performance obligation	$ 124,425
Expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Transaction price allocated to the remaining performance obligations
Remaining performance obligation	$ 203,833
Expected timing of satisfaction