CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Proved properties, accumulated depletion	$ 8,326,395,000	$ 8,237,186,000
Other property and equipment, accumulated depreciation and amortization	$ 49,108,000	$ 46,908,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares issued	120,107,896	118,245,320
Common stock, shares outstanding	120,107,896	118,245,320