REVENUE -Remaining Performance Obligations (Details) - Fixed-price contracts $ in Thousands	Dec. 31, 2019 USD ($)
Transaction price allocated to the remaining performance obligations
Remaining performance obligation	$ 494,904
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	$ 191,282
Expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Transaction price allocated to the remaining performance obligations
Remaining performance obligation	$ 116,900
Expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Transaction price allocated to the remaining performance obligations
Remaining performance obligation	$ 186,722
Expected timing of satisfaction