Derivative Instruments and Hedging Activities - Effect of Derivative Instruments on the Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Effect of derivative instruments on the Consolidated Balance Sheet
Derivative Assets	$ 3,412	$ 2,239
Derivatives Not Designated as Hedges | Commodity contracts
Effect of derivative instruments on the Consolidated Balance Sheet
Derivative Assets	3,412	2,239
Derivative Liabilities	9,589	30,637
Derivative instruments (current) | Derivatives Not Designated as Hedges | Commodity contracts
Effect of derivative instruments on the Consolidated Balance Sheet
Derivative Assets	0	0
Derivative Liabilities	9,589	30,637
Derivative instruments (non-current) | Derivatives Not Designated as Hedges | Commodity contracts
Effect of derivative instruments on the Consolidated Balance Sheet
Derivative Assets	3,412	2,239
Derivative Liabilities	$ 0	$ 0