Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation - Schedule of Amounts Recorded in Prepaid Expenses and Other Assets (Details)

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Basis of Presentation - Schedule of Amounts Recorded in Prepaid Expenses and Other Assets (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]    
Prepaid/accrued insurance $ 4,214 $ 2,401
Surety bond unamortized premiums 2,847 2,676
Prepaid deposits related to royalties 9,698 6,456
Advances for capital expenditures 860  
Derivative contract premiums 791  
Other 1,289 1,886
Prepaid expenses and other assets $ 19,699 $ 13,419