Derivative Instruments - Fair value gains (losses) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains (losses)	$ 57,019		$ (65,957)	$ 134,793	$ 458,459
Gains on settled marketing derivatives				78,098
Marketing derivative gains (losses)	(42)	$ (21,400)		94,081
Revenue
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains (losses)	57,019		$ (65,957)	134,793	$ 458,459
Marketing derivative gains (losses)	$ (42)			$ 94,081