CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
CONDENSED CONSOLIDATED BALANCE SHEETS
Natural gas and oil properties, using the full cost method, costs excluded from amortization	$ 1,130.4	$ 1,023.9
Common Stock, Par Value	$ 0.01	$ 0.01
Common Stock, Authorized, Shares	1,250,000,000	1,250,000,000
Common Stock, Issued, Shares	351,530,606	351,100,391
Common Stock in Treasury, Shares	14,223	64,715