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, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Extractive Industries [Abstract]
Net changes in prices and production costs	$ (11,365.5)	$ (2,697.8)	$ 267.8
Net changes in development costs	591.4	(2,317.3)	(3,456.8)
Sales and transfers of oil and gas produced, net of production costs	(1,876.9)	(4,076.0)	(3,972.4)
Net change due to extensions and discoveries	1,145.8	3,251.6	4,608.9
Net change due to purchases and sales of proved reserves	(287.4)	(1,041.0)	(135.6)
Development costs incurred	1,725.4	3,169.3	3,326.8
Accretion of discount	1,289.5	1,462.5	1,109.3
Revisions of previous quantity estimates	163.3	518.9	1,646.0
Net change in income taxes	2,568.3	790.3	(662.1)
Net increase (decrease)	(6,046.1)	(939.5)	2,731.9
Standardized measure	$ 3,859.1	$ 9,905.2	$ 10,844.7	$ 8,112.8