Supplemental Gas and Oil Information (Unaudited) Costs incurred (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Abstract]
Accrued capital costs included in development costs	$ 12.2
ARO additions and revisions	(1.2)
Costs incurred related to the development of proved undeveloped reserves	426.1	$ 606.5	$ 389.3
Proved property acquisitions	1.5	39.1	269.6
Unproved property acquisitions	2.0	25.8	532.4
Other acquisitions	0.0	0.8	13.2
Exploration costs (capitalized and expensed)	0.1	0.3	32.7
Development costs	556.2	1,133.1	1,189.3
Total costs incurred	$ 559.8	$ 1,199.1	$ 2,037.2