Supplemental Gas and Oil Information (Unaudited) Results of Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Results of Operations, Income before Income Taxes [Abstract]
Revenues	$ 1,271.0	$ 1,390.4	$ 2,374.6
Production costs	616.7	654.1	735.6
Exploration expenses	1.7	2.7	9.9
Depreciation, depletion and amortization	852.3	870.8	984.4
Impairment	1,194.3	55.6	1,143.2
Total expenses	2,665.0	1,583.2	2,873.1
Income (loss) before income taxes	(1,394.0)	(192.8)	(498.5)
Income tax benefit (expense)	517.2	70.6	182.5
Results of operations from producing activities excluding allocated corporate overhead and interest expenses	$ (876.8)	$ (122.2)	$ (316.0)