Supplemental Gas and Oil Information (Unaudited) Costs incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Abstract]
Accrued capital costs included in development costs	$ 34.6
ARO additions and revisions	23.5
Costs incurred related to the development of proved undeveloped reserves	258.1	$ 490.4	$ 792.9
Proved property acquisitions	431.6	49.6	465.4
Unproved property acquisitions	208.7	39.8	496.3
Exploration (capitalized and expensed)	13.4	8.7	23.6
Development	509.2	1,010.3	1,695.1
Total costs incurred	$ 1,162.9	$ 1,108.4	$ 2,680.4