Supplemental Gas and Oil Information (Unaudited) Change in Standardized Measure of Future Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in Standardized Measure of Future Cash Flows [Abstract]
Beginning Balance	$ 5,340.0	$ 4,383.9	$ 3,034.7
Sales of gas, oil and NGL produced during the period, net of production costs	(736.3)	(1,639.0)	(1,317.9)
Net change in sales prices and in production (lifting) costs related to future production	(6,307.8)	726.6	1,236.3
Net change due to extensions, discoveries and improved recovery	1,765.7	979.9	2,230.7
Net change due to revisions of quantity estimates	(1,350.2)	35.9	(709.6)
Changes due to purchases of reserves in place	29.7	695.3	36.8
Changes due to sales of reserves in place	(48.8)	(1,153.7)	(73.2)
Previously estimated development costs incurred during the period	865.0	867.5	722.7
Changes in estimated future development costs	560.7	409.6	(596.5)
Accretion of discount	752.9	597.3	402.2
Net change in income taxes	1,554.4	(600.3)	(601.7)
Other	51.0	37.0	19.4
Net change	(2,863.7)	956.1	1,349.2
Ending Balance	$ 2,476.3	$ 5,340.0	$ 4,383.9