Supplemental Gas and Oil Information (Unaudited) Change in Standardized Measure of Future Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in Standardized Measure of Future Cash Flows [Abstract]
Beginning Balance	$ 2,676.3	$ 4,955.7	$ 3,097.3
Sales of gas, oil and NGL produced during the period, net of production costs	(428.1)	(838.7)	(1,413.0)
Net change in sales prices and in production (lifting) costs related to future production	(2,136.4)	(1,988.6)	1,632.5
Net change due to extensions and discoveries	2.4	220.9	692.6
Net change due to revisions of quantity estimates	159.6	(2,079.2)	732.0
Changes due to purchases of reserves in place	0.0	34.2	117.0
Changes due to sales of reserves in place	(1.9)	(617.8)	(369.6)
Previously estimated development costs incurred during the period	256.1	460.8	735.6
Changes in estimated future development costs	418.7	1,064.7	(28.3)
Accretion of discount	310.7	622.8	375.4
Net change in income taxes	277.9	841.5	(615.7)
Other	0.0	0.0	(0.1)
Net change	(1,141.0)	(2,279.4)	1,858.4
Ending Balance	$ 1,535.3	$ 2,676.3	$ 4,955.7