Supplemental Oil and Gas Information (Principal Sources of Change in the Standardized Measure of Discounted Future Net Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Oil and Gas Information [Abstract]
Net changes in prices and production costs	$ 2,428.4	$ (1,476.1)	$ (11,365.5)
Net changes in development costs	(724.4)	544.9	591.4
Sales and transfers of oil and gas produced, net of production costs	(1,576.0)	(1,196.3)	(1,876.9)
Net change due to extensions and discoveries	807.9	280.5	1,145.8
Net change due to purchases and sales of proved reserves	85.9	(583.4)	(287.4)
Development costs incurred	802.7	479.6	1,725.4
Accretion of discount	270.9	428.1	1,289.5
Revisions of previous quantity estimates	(109.5)	(49.2)	163.3
Net change in income taxes	(643.0)	292.8	2,568.3
Net increase (decrease)	1,342.9	(1,279.1)	(6,046.1)
Standardized measure	$ 3,922.9	$ 2,580.0	$ 3,859.1	$ 9,905.2