Derivative Instruments Derivative Instruments - Gross Amounts of Recognized Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Commodity derivative assets, Gross amounts of recognized assets	$ 2,603	$ 4,061
Commodity derivative assets, Gross amounts offset on balance sheet	0	0
Derivative assets, Net amounts of assets on balance sheet	2,603	4,061
Commodity derivative liability, Gross amounts of recognized liabilities	0	(59,489)
Commodity derivative liability, Gross amounts offset on balance sheet	0	0
Derivative liability, Net amounts of liabilities on balance sheet	$ 0	$ (59,489)