Supplemental Gas and Oil Information (Unaudited) Change in Standardized Measure of Future Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Standardized Measure of Future Cash Flows [Abstract]
Beginning Balance	$ 1,928.0	$ 2,476.3	$ 5,340.0
Sales of gas, oil and NGL produced during the period, net of production costs	(872.7)	(654.3)	(736.3)
Net change in sales prices and in production (lifting) costs related to future production	1,457.2	(739.4)	(6,307.8)
Net change due to extensions and discoveries	556.8	81.8	1,765.7
Net change due to revisions of quantity estimates	9.9	122.7	(1,350.2)
Changes due to purchases of reserves in place	342.7	256.5	29.7
Changes due to sales of reserves in place	(504.7)	(4.3)	(48.8)
Previously estimated development costs incurred during the period	475.4	374.6	865.0
Changes in estimated future development costs	(283.4)	(476.5)	560.7
Accretion of discount	235.7	311.1	752.9
Net change in income taxes	(227.4)	205.4	1,554.4
Other	(20.2)	(25.9)	51.0
Net change	1,169.3	(548.3)	(2,863.7)
Ending Balance	$ 3,097.3	$ 1,928.0	$ 2,476.3