Annual report pursuant to Section 13 and 15(d)

REVENUE, CONTRACT ASSETS AND LIABILITIES AND OTHER CONTRACT MATTERS - Significant Customers (Details)

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REVENUE, CONTRACT ASSETS AND LIABILITIES AND OTHER CONTRACT MATTERS - Significant Customers (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Revenue $ 60,231 $ 49,712 $ 54,014 $ 57,290 $ 37,277 $ 49,884 $ 45,868 $ 37,993 $ 221,247 $ 171,022 $ 286,326
Sales Revenue, Net | Customer Concentration Risk | Customer A                      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Revenue                 49,123 21,781  
Sales Revenue, Net | Customer Concentration Risk | Customer B                      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Revenue                 25,873    
Sales Revenue, Net | Customer Concentration Risk | Customer C                      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Revenue                 $ 23,279    
Sales Revenue, Net | Customer Concentration Risk | Customer D                      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Revenue                   $ 44,724  
Sales Revenue, Net | Customer Concentration Risk | Customer E                      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]                      
Revenue                     $ 65,981,000