Quarterly report pursuant to Section 13 or 15(d)

Derivative Financial Instruments - Cash Receipts on Derivative Settlements, Net Included within Net Cash Provided by (Used in) Operating Activities (Details)

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Derivative Financial Instruments - Cash Receipts on Derivative Settlements, Net Included within Net Cash Provided by (Used in) Operating Activities (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Cash receipts on derivative settlements, net $ 3,924 $ 4,746