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FAIR VALUE MEASUREMENTS - Estimated FV (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Sep. 07, 2017
Feb. 16, 2017
Fair value measurements        
Principal amount $ 625,005 $ 425,005    
6.75% senior notes due 2025        
Fair value measurements        
Interest rate (as a percent) 6.75% 6.75% 6.75% 6.75%
Principal amount     $ 850,000 $ 850,000
Total        
Fair value measurements        
Estimated fair value of debt $ 584,380 $ 443,790