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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Oil and Gas Information [Abstract]
Net changes in prices and production costs	[1]	$ 2,972.6	$ 2,428.4	$ (1,476.1)
Net changes in development costs	[1]	(1,891.1)	(724.4)	544.9
Sales and transfers of oil and gas produced, net of production costs	[1]	(1,978.6)	(1,576.0)	(1,196.3)
Net change due to extensions and discoveries	[1]	1,930.3	807.9	280.5
Net change due to purchases and sales of proved reserves	[1]	3,152.4	85.9	(583.4)
Development costs incurred	[1]	1,017.3	802.7	479.6
Accretion of discount	[1]	469.5	270.9	428.1
Revisions of previous quantity estimates	[1]	(347.8)	(109.5)	(49.2)
Net change in income taxes	[1]	(967.6)	(643.0)	292.8
Net increase (decrease)	[1]	4,357.0	1,342.9	(1,279.1)
Standardized measure	[1]	$ 8,279.9	$ 3,922.9	$ 2,580.0	$ 3,859.1

[1] 2018 includes noncontrolling interest in MP GOM.