Supplemental Gas and Oil Information (Unaudited) Capitalized costs (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Capitalized Costs, Oil and Gas Producing Activities, Net [Abstract]
Proved properties	$ 13,314.9	$ 12,278.7
Unproved properties, net	691.0	825.2
Total proved and unproved properties	14,005.9	13,103.9
Accumulated depreciation, depletion and amortization	(6,870.2)	(6,153.0)
Net capitalized costs	$ 7,135.7	$ 6,950.9