Supplemental Oil and Gas Information (Unaudited) - Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 14,594.2	$ 28,533.4	$ 34,423.0
Future development costs	(1,694.1)	(4,861.8)	(5,159.2)
Future production costs	(8,412.1)	(11,566.0)	(12,785.9)
Future income taxes	(166.8)	(2,137.9)	(2,924.9)
Future net cash flows	4,321.2	9,967.7	13,553.0
10% annual discount for estimated timing of cash flows	(1,702.6)	(4,140.1)	(5,273.1)
Standardized measure of discounted future net cash flows	2,618.6	5,827.6	8,279.9	$ 3,922.9
U.S.
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	9,976.7	23,565.6	23,473.9
Future development costs	(1,289.8)	(4,137.8)	(3,279.1)
Future production costs	(5,777.5)	(8,986.2)	(7,279.5)
Future income taxes	0.0	(1,709.3)	(2,216.5)
Future net cash flows	2,909.4	8,732.3	10,698.8
10% annual discount for estimated timing of cash flows	(1,079.2)	(3,633.1)	(4,295.4)
Standardized measure of discounted future net cash flows	1,830.2	5,099.2	6,403.4
Canada
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	4,617.5	4,912.1	5,437.5
Future development costs	(404.3)	(723.7)	(1,362.7)
Future production costs	(2,634.6)	(2,549.9)	(2,693.0)
Future income taxes	(166.8)	(414.5)	(236.4)
Future net cash flows	1,411.8	1,224.0	1,145.4
10% annual discount for estimated timing of cash flows	(623.4)	(504.0)	(531.4)
Standardized measure of discounted future net cash flows	788.4	720.0	614.0
Malaysia and Other
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	0.0	55.7	5,511.6
Future development costs	0.0	(0.3)	(517.4)
Future production costs	0.0	(29.9)	(2,813.4)
Future income taxes	0.0	(14.1)	(472.0)
Future net cash flows	0.0	11.4	1,708.8
10% annual discount for estimated timing of cash flows	0.0	(3.0)	(446.3)
Standardized measure of discounted future net cash flows	$ 0.0	$ 8.4	$ 1,262.5