Income taxes (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Significant components of deferred tax assets
Derivative financial instruments	$ 13,476	$ 7,108
Oil and natural gas properties and equipment	(191,379)	(173,279)
Net operating loss carry-forward	233,969	222,017
Accrued bonus	1,201	3,502
Other	4,174	3,347
Gross deferred tax assets	61,441	62,695
Valuation allowance	(75)	(66)
Net deferred tax assets	61,366	62,629
Net deferred tax assets and liabilities
Deferred tax asset	61,366	62,629
Net deferred tax assets	$ 61,366	$ 62,629