CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Statement of Financial Position [Abstract]
Proved properties, accumulated depletion (in dollars)	$ 6,838,561,000	$ 6,668,012,000
Other property and equipment, accumulated depreciation and amortization (in dollars)	$ 47,300,000	$ 42,432,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, shares issued	114,400,776	113,594,788
Common stock, shares outstanding	114,400,776	113,594,788