Annual report pursuant to Section 13 and 15(d)

IMPAIRMENT OF LONG???LIVED ASSETS (Details)

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IMPAIRMENT OF LONG‑LIVED ASSETS (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]        
Property and equipment impairment expense $ 0 $ 0 $ 6,305,000  
Net carrying value of property and equipment $ 912,846,000 $ 470,910,000    
Drilling        
Segment Reporting Information [Line Items]        
Property and equipment impairment expense     6,300,000  
Net carrying value of property and equipment       $ 15,000,000
Drilling | Significant other observable inputs (Level 2)        
Segment Reporting Information [Line Items]        
Fair value of the reporting unit     $ 8,700,000