Supplemental Oil and Gas Information - Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) $ / Mcf	Dec. 31, 2018 USD ($) $ / Mcf	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 28,533.4	$ 34,423.0	$ 21,992.1
Future development costs	(4,861.8)	(5,159.2)	(3,793.5)
Future production costs	(11,566.0)	(12,785.9)	(9,577.7)
Future income taxes	(2,137.9)	(2,924.9)	(1,287.0)
Future net cash flows	9,967.7	13,553.0	7,333.9
10% annual discount for estimated timing of cash flows	(4,140.1)	(5,273.1)	(3,411.0)
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 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
Increase Decrease Due To Purchases And Sales Of Minerals In Place	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)
Standardized Measure of Discounted Future Net Cash Flow of Proved Oil and Gas Reserves, Period Increase (Decrease)	(2,452.3)	4,357.0	1,342.9
Standardized measure of discounted future net cash flows	5,827.6	8,279.9	3,922.9	$ 2,580.0
United States
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	23,565.6	23,473.9	12,885.8
Future development costs	(4,137.8)	(3,279.1)	(2,079.5)
Future production costs	(8,986.2)	(7,279.5)	(4,765.3)
Future income taxes	(1,709.3)	(2,216.5)	(893.7)
Future net cash flows	8,732.3	10,698.8	5,147.3
10% annual discount for estimated timing of cash flows	(3,633.1)	(4,295.4)	(2,698.2)
Standardized measure of discounted future net cash flows	5,099.2	6,403.4	2,449.1
Canada
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	4,912.1	5,437.5	4,714.3
Future development costs	(723.7)	(1,362.7)	(1,081.7)
Future production costs	(2,549.9)	(2,693.0)	(2,507.4)
Future income taxes	(414.5)	(236.4)	(161.1)
Future net cash flows	1,224.0	1,145.4	964.1
10% annual discount for estimated timing of cash flows	(504.0)	(531.4)	(394.6)
Standardized measure of discounted future net cash flows	720.0	614.0	569.5
Malaysia & Other
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	55.7	5,511.6	4,392.0
Future development costs	(0.3)	(517.4)	(632.3)
Future production costs	(29.9)	(2,813.4)	(2,305.0)
Future income taxes	(14.1)	(472.0)	(232.2)
Future net cash flows	11.4	1,708.8	1,222.5
10% annual discount for estimated timing of cash flows	(3.0)	(446.3)	(318.2)
Standardized measure of discounted future net cash flows	$ 8.4	$ 1,262.5	$ 904.3
Net natural gas revenue
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Average production costs per volume of natural gas (in dollars per cubic foot) | $ / Mcf	2.57	3.10