Costs of Oil and Gas Properties (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Oil and Gas Property, Successful Effort Method, Gross [Abstract]
Proved properties	$ 2,338,303	$ 2,317,053
Unproved properties	74,921	86,224
Total capitalized costs	2,413,224	2,403,277
Accumulated depletion	(1,312,963)	(1,282,989)
Net capitalized costs	$ 1,100,261	$ 1,120,288