Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, par value	$ 0.01	$ 0.01
Unevaluated costs of oil and gas properties	$ 304,340	$ 307,267
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	182,882,012	179,015,118
Treasury stock, shares	6,288,230	5,921,102