Derivative Instruments - Fair value gains (losses) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains (losses)		$ (87,594)	$ 658,283	$ (514,181)
Gains on settled marketing derivatives		72,687
Marketing derivative fair value gains (losses)		94,081	(21,394)
Commodity derivative fair value gains (losses) on derivatives monetized prior to settlement dates		370,000	750,000
Premium paid for a collar agreement		13,000
Derivative liabilities
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains (losses)		73,000	94,000
Marketing derivative fair value gains (losses)		73,000
Revenue
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains (losses)		(87,594)	658,283	$ (514,181)
Marketing derivative fair value gains (losses)		$ 94,081	$ (21,394)
Commodity derivative fair value gains (losses)
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains (losses)	$ 21,000