Supplemental Gas and Oil Information (Unaudited) Results of Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Results of Operations, Income before Income Taxes [Abstract]
Revenues	$ 1,390.4	$ 2,374.6	$ 1,901.2
Production costs	654.1	735.6	583.3
Exploration expenses	2.7	9.9	11.9
Depreciation, depletion and amortization	870.8	984.4	954.2
Abandonment and impairment	55.6	1,143.2	93.0
Total expenses	1,583.2	2,873.1	1,642.4
Income before income taxes	(192.8)	(498.5)	258.8
Income taxes	70.6	182.5	(96.3)
Results of operations from producing activities excluding allocated corporate overhead and interest expenses	$ (122.2)	$ (316.0)	$ 162.5