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CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Assets    
Accumulated depreciation $ 377,063 $ 340,423
Preferred stock    
Par value (in dollars per share) $ 0.01 $ 0.01
Shares authorized (in shares) 10,000,000 10,000,000
Shares issued (in shares) 5,750,000 5,750,000
Shares outstanding (in shares) 5,750,000 5,750,000
Common stock    
Par value (in dollars per share) $ 0.01 $ 0.01
Shares authorized (in shares) 250,000,000 250,000,000
Shares issued (in shares) 178,282,370 178,255,843
Shares outstanding (in shares) 178,282,370 178,255,843