Supplemental Gas and Oil Information (Unaudited) Change in Standardized Measure of Future Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in Standardized Measure of Future Cash Flows [Abstract]
Beginning Balance	$ 2,476.3	$ 5,340.0	$ 4,383.9
Sales of gas, oil and NGL produced during the period, net of production costs	(654.3)	(736.3)	(1,639.0)
Net change in sales prices and in production (lifting) costs related to future production	(739.4)	(6,307.8)	726.6
Net change due to extensions and discoveries	81.8	1,765.7	979.9
Net change due to revisions of quantity estimates	122.7	(1,350.2)	35.9
Changes due to purchases of reserves in place	256.5	29.7	695.3
Changes due to sales of reserves in place	(4.3)	(48.8)	(1,153.7)
Previously estimated development costs incurred during the period	374.6	865.0	867.5
Changes in estimated future development costs	(476.5)	560.7	409.6
Accretion of discount	311.1	752.9	597.3
Net change in income taxes	205.4	1,554.4	(600.3)
Other	(25.9)	51.0	37.0
Net change	(548.3)	(2,863.7)	956.1
Ending Balance	$ 1,928.0	$ 2,476.3	$ 5,340.0