Investment in Oil and Gas Properties (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Costs incurred during the year
Lease operating expenses	$ 179,475	$ 152,326	$ 156,786
Production taxes	9,380	5,808	7,920
Accretion expense	30,764	34,469	39,306
United States [Member]
Costs incurred during the year
Lease operating expenses	179,475	152,326	156,786
Production taxes	9,380	5,808	7,920
Accretion expense	30,764	34,469	39,306
Expensed costs	$ 219,619	$ 192,603	$ 204,012