CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Statement of Financial Position [Abstract]
Proved properties, accumulated depletion	$ 8,237,186	$ 6,901,997
Other property and equipment, accumulated depreciation and amortization	$ 46,908	$ 47,989
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares issued	118,245,320	114,525,673
Common stock, shares outstanding	118,245,320	114,525,673