Capitalized costs and AD&D related to oil and natural gas producing activities (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Subject to amortization	$ 2,531,562	$ 2,345,114	$ 2,138,565
Not subject to amortization	191,794	232,462	182,402
Total capitalized costs	2,723,356	2,577,576	2,320,967
Less accumulated depreciation, depletion and amortization	1,383,386	1,229,654	1,093,723
Net capitalized costs	$ 1,339,970	$ 1,347,922	$ 1,227,244