CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Statement of Financial Position [Abstract]
Proved properties, accumulated depletion (in dollars)	$ 7,423,306,000	$ 6,901,997,000
Other property and equipment, accumulated depreciation and amortization (in dollars)	$ 49,616,000	$ 47,989,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares issued	118,239,252	114,525,673
Common stock, shares outstanding	118,239,252	114,525,673