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Derivative Instruments Summary of Outstanding Contracts with Respect to Diesel Fuel (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative [Line Items]        
Non-cash gain (loss) on derivatives, net     $ (11,465) $ 68,420
Gain on natural gas derivatives, net $ (12,685) $ 28,022 (2,511) 74,880
Diesel Fuel [Member]        
Derivative [Line Items]        
Cash received (paid) on derivatives, net 0 185 0 919
Non-cash gain (loss) on derivatives, net 0 (1,098) 0 (3,729)
Gain on natural gas derivatives, net $ 0 $ (913) $ 0 $ (2,810)