CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Proved properties, accumulated depletion	$ 8,480,853,000	$ 8,460,589,000
Other property and equipment, accumulated depreciation and amortization	$ 46,991,000	$ 50,058,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares issued	119,099,106	119,399,983
Common stock, shares outstanding	119,099,106	119,399,983