Costs of Oil and Gas Properties (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Oil and Gas Property, Successful Effort Method, Gross [Abstract]
Proved properties	$ 2,482,185	$ 2,021,181
Unproved properties	88,618	81,904
Total capitalized costs	2,570,803	2,103,085
Accumulated depletion	(1,234,626)	(1,095,197)
Net capitalized costs	$ 1,336,177	$ 1,007,888