Supplemental Gas and Oil Information (Unaudited) Costs incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Abstract]
Accrued capital costs included in development costs	$ 127.6
Costs Incurred, Asset Retirement Obligation Incurred	21.0
Costs Incurred To Advance The Development Of Proved Undeveloped Reserves	811.3	$ 796.7	$ 645.9
Unproved	39.8	496.3	9.3
Proved	49.6	465.4	31.6
Exploration	8.7	23.6	14.6
Development	1,010.3	1,695.1	1,440.8
Total costs incurred	$ 1,108.4	$ 2,680.4	$ 1,496.3