Derivative Instruments - Fair value gains (losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains	$ 220,788	$ 57,019	$ 471,847	$ 134,793
Marketing derivative fair value gains (losses)	0	(42)	0	94,081
Revenue
Summary of realized and unrealized gains (losses) on derivative instruments
Commodity derivative fair value gains	$ 220,788	57,019	$ 471,847	134,793
Marketing derivative fair value gains (losses)		(42)		$ 94,081
Maximum
Summary of realized and unrealized gains (losses) on derivative instruments
Marketing derivative fair value gains (losses)		$ 1,000