CONTRACT REVENUE AND PERCENTAGE-OF-COMPLETION METHOD - Revenues from Major Customers (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Major Customer [Line Items]
Revenue	$ 37,277	$ 49,884	$ 45,868	$ 37,993	$ 55,461	$ 65,384	$ 81,502	$ 83,979	$ 171,022	$ 286,326	$ 306,120
Customer A
Revenue, Major Customer [Line Items]
Revenue									44,724
Customer B
Revenue, Major Customer [Line Items]
Revenue									$ 21,781
Customer C
Revenue, Major Customer [Line Items]
Revenue										$ 65,981
Customer D
Revenue, Major Customer [Line Items]
Revenue											55,775
Customer E
Revenue, Major Customer [Line Items]
Revenue											$ 36,320