Quarterly report pursuant to Section 13 or 15(d)

Property and Equipment (Schedule of Asset Retirement Obligation) (Details)

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Property and Equipment (Schedule of Asset Retirement Obligation) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset retirement obligation, beginning of period     $ 75,100 $ 34,276  
Liabilities incurred     909 9,505  
Liabilities settled     (719) (344)  
Accretion expense $ 1,015 $ 410 2,019 692  
Revisions in estimated cash flows     (374) 0  
Asset retirement obligation as of end of period 76,935 44,129 76,935 44,129  
Less current portion 120 195 120 195 $ 120
Asset retirement obligation, long-term $ 76,815 $ 43,934 $ 76,815 $ 43,934 $ 74,980