Investment in Oil and Gas Properties (Details 2) (Evaluated Costs [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Evaluated Costs [Member]
Net costs evaluated
Acquisition costs evaluated during the year	$ (2,397)	$ 42,664	$ (203,776)
Exploration costs evaluated during the year	(51,207)	10,491	15,337
Capitalized interest costs evaluated during the year	42,033	30,783	23,943
Total costs incurred during the year, net of divestitures	$ (11,571)	$ 83,938	$ (164,496)