CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Proved properties, accumulated depletion	$ 8,460,589	$ 8,237,186
Other property and equipment, accumulated depreciation and amortization	$ 50,058	$ 46,908
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares issued	119,399,983	118,245,320
Common stock, shares outstanding	119,399,983	118,245,320