Quarterly report pursuant to Section 13 or 15(d)

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

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Derivative Financial Instruments - Cash Receipts (Payments) on Derivative Settlements, Net Included within Net Cash Provided by Operating Activities (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Cash receipts (payments) on derivative settlements, net $ (1,149) $ 2,208