Income taxes - Net deferred tax liability (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Significant components of deferred tax assets
Net operating loss carryforward	$ 392,276	$ 355,100
Oil and natural gas properties, midstream service assets and other fixed assets	(168,031)	(80,153)
Stock-based compensation	19,845	14,025
Derivatives, liability	(8,188)
Derivatives, asset		3,788
Gain (loss) on sale of assets	(7,693)	40,177
Other	3,997	8,465
Net deferred tax asset before valuation allowance	232,206	341,402
Valuation allowance	(237,262)	(341,402)
Net deferred tax liability	$ (5,056)	$ 0