Supplemental Oil and Gas Information (Unaudited) - Costs Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property acquisition costs
Unproved	$ 8.5	$ 1.8	$ 8.8
Proved	27.1	128.5
Proved			0.5
Total acquisition costs	35.6	130.3	8.3
Exploration costs	198.1	113.3	62.2
Development costs	880.8	917.7	619.3
Total costs incurred	1,114.5	1,161.3	689.8
Charged to expense
Dry hole expense	169.8	82.1	17.3
Geophysical and other costs	54.1	37.7	32.8
Total charged to expense	223.9	119.8	50.1
Property additions	890.6	1,041.5	$ 639.7
Terra Nova
Charged to expense
Additional percentage acquired through commercial settlement			7.525%
U.S.
Property acquisition costs
Unproved	0.0	1.8	$ 8.8
Proved	12.8	128.5	19.9
Total acquisition costs	12.8	130.3	28.7
Exploration costs	157.8	42.2	31.7
Development costs	667.2	704.9	513.2
Total costs incurred	837.8	877.4	573.6
Charged to expense
Dry hole expense	153.1	23.0	17.3
Geophysical and other costs	13.4	15.8	13.1
Total charged to expense	166.5	38.8	30.4
Property additions	671.3	838.6	543.2
Canada
Property acquisition costs
Unproved	0.0	0.0	0.0
Proved	0.0	0.0
Proved			20.4
Total acquisition costs	0.0	0.0	20.4
Exploration costs	0.4	0.8	0.4
Development costs	206.2	208.5	102.4
Total costs incurred	206.6	209.3	82.4
Charged to expense
Dry hole expense	0.0	0.0	0.0
Geophysical and other costs	0.4	0.8	0.4
Total charged to expense	0.4	0.8	0.4
Property additions	206.2	208.5	82.0
Other
Property acquisition costs
Unproved	8.5	0.0	0.0
Proved	14.3	0.0	0.0
Total acquisition costs	22.8	0.0	0.0
Exploration costs	39.9	70.3	30.1
Development costs	7.4	4.3	3.7
Total costs incurred	70.1	74.6	33.8
Charged to expense
Dry hole expense	16.7	59.1	0.0
Geophysical and other costs	40.3	21.1	19.3
Total charged to expense	57.0	80.2	19.3
Property additions	$ 13.1	$ (5.6)	$ 14.5