Income taxes (Details 4) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Significant components of deferred tax assets
Oil and natural gas properties, midstream service assets and other fixed assets	$ (424,712)	$ (278,735)
Net operating loss carry-forward	353,724	284,890
Derivatives	(121,365)	(30,859)
Stock-based compensation	10,718	6,578
Accrued bonus	3,256	3,740
Capitalized interest	3,049	2,099
Other	(316)	(240)
Gross deferred tax liability	(175,646)	(12,527)
Valuation allowance	(1,299)	(132)
Net deferred tax assets and liabilities
Deferred tax asset	0	3,634
Deferred tax liability	(176,945)	(16,293)
Net deferred tax liability	(176,945)	(12,659)
Deferred Tax Liabilities, Net	$ 176,945	$ 12,659