Supplemental Gas and Oil Information (Unaudited) Change in Standardized Measure of Future Cash Flows (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in Standardized Measure of Future Cash Flows [Abstract]
Beginning Balance	$ 4,383.9	$ 3,034.7	$ 3,525.6
Sales of gas, oil and NGL produced during the period, net of production costs	(1,639.0)	(1,317.9)	(892.3)
Net change in sales prices and in production (lifting) costs related to future production	726.6	1,236.3	(2,083.5)
Net change due to extensions, discoveries and improved recovery	979.9	2,230.7	948.5
Net change due to revisions of quantity estimates	35.9	(709.6)	(387.8)
Changes due to purchases of reserves in place	695.3	36.8	831.4
Changes due to sales of reserves in place	(1,153.7)	(73.2)	0
Previously estimated development costs incurred during the period	867.5	722.7	513.0
Changes in estimated future development costs	409.6	(596.5)	(209.3)
Accretion of discount	597.3	402.2	499.4
Net change in income taxes	(600.3)	(601.7)	273.6
Other	37.0	19.4	16.1
Net change	956.1	1,349.2	(490.9)
Ending Balance	$ 5,340.0	$ 4,383.9	$ 3,034.7