Supplemental Gas and Oil Information (Unaudited) Capitalized costs (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Capitalized Cost [Abstract]
Proved properties	$ 10,234.3	$ 8,172.4
Unproved properties, net	937.9	326.8
Total proved and unproved properties	11,172.2	8,499.2
Accumulated depreciation, depletion and amortization	(4,258.1)	(3,339.2)
Net capitalized costs	$ 6,914.1	$ 5,160.0