Supplemental Gas and Oil Information (Unaudited) Costs incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Abstract]
Accrued capital costs included in development costs	$ 60.6
ARO additions and revisions	32.0
Costs incurred related to the development of proved undeveloped reserves	389.3	$ 258.1	$ 490.4
Proved property acquisitions	269.6	431.6	49.6
Unproved property acquisitions	532.4	208.7	39.8
Other acquisitions	13.2	0.0	0.0
Exploration costs (capitalized and expensed)	32.7	13.4	8.7
Development costs	1,189.3	509.2	1,010.3
Total costs incurred	$ 2,037.2	$ 1,162.9	$ 1,108.4