Derivative Instruments - Assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Commodity derivative
Commodity derivative assets
Gross amounts on balance sheet	$ 658,830	$ 1,367,554
Gross amounts offset on balance sheet	(51,398)	(65,612)
Total asset derivatives	607,432	1,301,942
Commodity derivative liabilities
Gross amounts on balance sheet	(51,930)	(72,901)
Gross amounts offset on balance sheet	51,398	65,612
Total liability derivatives	$ (532)	(7,289)
Marketing derivative
Commodity derivative assets
Gross amounts on balance sheet		311,083
Gross amounts offset on balance sheet		(311,083)
Commodity derivative liabilities
Gross amounts on balance sheet		(332,477)
Gross amounts offset on balance sheet		311,083
Total liability derivatives		$ (21,394)

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