Supplemental Oil and Gas Information (Unaudited) - Principal Sources of Change in the Standardized Measure of Discounted Future Net Cash Flows (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) $ / bbl	Dec. 31, 2019 USD ($) $ / bbl	Dec. 31, 2018 USD ($) $ / bbl
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Net changes in prices and production costs	$ (5,942.1)	$ (2,993.9)	$ 2,972.6
Net changes in development costs	2,215.1	(675.7)	(1,891.1)
Sales and transfers of oil and natural gas produced, net of production costs	(1,123.1)	(2,163.8)	(1,978.6)
Net change due to extensions and discoveries	568.5	1,221.9	1,930.3
Net change due to purchases and sales of proved reserves	(14.6)	(628.1)	3,152.4
Development costs incurred	736.8	1,282.4	1,017.3
Accretion of discount	699.3	1,002.0	469.5
Revisions of previous quantity estimates	(1,461.3)	(71.2)	(347.8)
Net change in income taxes	1,112.4	574.1	(967.6)
Net increase (decrease)	(3,209.0)	(2,452.3)	4,357.0
Standardized measure, beginning balance	5,827.6	8,279.9	3,922.9
Standardized measure, ending balance	$ 2,618.6	$ 5,827.6	$ 8,279.9
Net crude oil and condensate revenue
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Average production costs per barrel of oil (in dollars per barrel) | $ / bbl	39.57	55.69	65.56
Net natural gas revenue
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Average production costs per volume of natural gas (in dollars per cubic foot) | $ / bbl	1.98	2.57	3.10