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, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Extractive Industries [Abstract]
Net changes in prices and production costs	$ (1,476.1)	$ (11,365.5)	$ (2,697.8)
Net changes in development costs	544.9	591.4	(2,317.3)
Sales and transfers of oil and gas produced, net of production costs	(1,196.3)	(1,876.9)	(4,076.0)
Net change due to extensions and discoveries	280.5	1,145.8	3,251.6
Net change due to purchases and sales of proved reserves	(583.4)	(287.4)	(1,041.0)
Development costs incurred	479.6	1,725.4	3,169.3
Accretion of discount	428.1	1,289.5	1,462.5
Revisions of previous quantity estimates	(49.2)	163.3	518.9
Net change in income taxes	292.8	2,568.3	790.3
Net increase (decrease)	(1,279.1)	(6,046.1)	(939.5)
Standardized measure	$ 2,580.0	$ 3,859.1	$ 9,905.2	$ 10,844.7