Income taxes (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Significant components of deferred tax assets
Derivative financial instruments	$ 4,403	$ 7,108
Oil and natural gas properties and equipment	(226,192)	(175,823)
Net operating loss carry-forward	252,173	222,017
Accrued bonus	2,722	3,502
Stock-based compensation	3,957	2,928
Other	4,061	2,963
Gross deferred tax assets	41,124	62,695
Valuation allowance	(96)	(66)
Net deferred tax assets	41,028	62,629
Net deferred tax assets and liabilities
Deferred tax asset	41,028	62,629
Deferred tax liability	0	0
Net deferred tax assets	$ 41,028	$ 62,629