Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Statement of Financial Position [Abstract]
Unproved property balance	$ 92,579	$ 84,146
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value per share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	43,450,367	42,969,546
Common stock, shares outstanding	42,930,071	42,485,075
Treasury stock shares held, at cost	520,296	484,471