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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Net changes in prices and production costs	$ (2,993.9)	$ 2,972.6	$ 2,428.4
Net changes in development costs	(675.7)	(1,891.1)	(724.4)
Sales and transfers of oil and natural gas produced, net of production costs	(2,163.8)	(1,978.6)	(1,576.0)
Net change due to extensions and discoveries	1,221.9	1,930.3	807.9
Net change due to purchases and sales of proved reserves	(628.1)	3,152.4	85.9
Development costs incurred	1,282.4	1,017.3	802.7
Accretion of discount	1,002.0	469.5	270.9
Revisions of previous quantity estimates	(71.2)	(347.8)	(109.5)
Net change in income taxes	574.1	(967.6)	(643.0)
Net increase (decrease)	(2,452.3)	4,357.0	1,342.9
Standardized measure	$ 5,827.6	$ 8,279.9	$ 3,922.9	$ 2,580.0