Derivative Instruments - Location and Fair Value of Commodity Derivatives (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Derivative assets, gross amounts of recognized assets	$ 353	$ 482
Derivative assets, gross amounts offset in the balance sheet	0	(120)
Derivative assets	353	362
Derivative liabilities, gross amounts of recognized liabilities	137,664	81,002
Derivative liabilities, gross amounts offset in the balance sheet	(2,097)	(8,819)
Derivative liabilities	135,567	72,183
Current assets \| Commodity derivative instruments
Offsetting Assets [Line Items]
Derivative assets, gross amounts of recognized assets	344	482
Derivative assets, gross amounts offset in the balance sheet	0	(120)
Derivative assets	344	362
Non-current assets \| Commodity derivative instruments
Offsetting Assets [Line Items]
Derivative assets, gross amounts of recognized assets	9
Derivative assets, gross amounts offset in the balance sheet	0
Derivative assets	9
Current liabilities \| Commodity derivative instruments
Offsetting Assets [Line Items]
Derivative liabilities, gross amounts of recognized liabilities	117,629	66,838
Derivative liabilities, gross amounts offset in the balance sheet	(1,913)	(6,369)
Derivative liabilities	115,716	60,469
Non-current liabilities \| Commodity derivative instruments
Offsetting Assets [Line Items]
Derivative liabilities, gross amounts of recognized liabilities	20,035	14,164
Derivative liabilities, gross amounts offset in the balance sheet	(184)	(2,450)
Derivative liabilities	$ 19,851	$ 11,714

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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