CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Proved properties, accumulated depletion	$ 8,575,255	$ 8,460,589
Other property and equipment, accumulated depreciation and amortization	$ 45,265	$ 50,058
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares issued	119,347,173	119,399,983
Common stock, shares outstanding	119,347,173	119,399,983