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, 2022	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Property acquisition costs
Unproved	$ 1.8	$ 8.8	$ 14.3
Proved	128.5	(0.5)	0.2
Total acquisition costs	130.3	8.3	14.5
Refunded exploration costs			58.6
Exploration costs	113.3	62.2
Development costs	917.7	619.3	736.8
Total costs incurred	1,161.3	689.8	809.9
Charged to expense
Dry hole expense	82.1	17.3
Geophysical and other costs	37.7	32.8	38.6
Total charged to expense	119.8	50.1	38.6
Property additions	$ 1,041.4	639.7	771.3
Terra Nova
Charged to expense
Additional percentage acquired through commercial settlement	7.525%			7.525%
U.S.
Property acquisition costs
Unproved	$ 1.8	8.8	6.5
Proved	128.5	19.9	0.2
Total acquisition costs	130.3	28.7	6.7
Refunded exploration costs			34.3
Exploration costs	42.2	31.7
Development costs	704.9	513.2	609.2
Total costs incurred	877.4	573.6	650.2
Charged to expense
Dry hole expense	23.0	17.3
Geophysical and other costs	15.8	13.1	14.3
Total charged to expense	38.8	30.4	14.3
Property additions	838.6	543.2	635.9
Canada
Property acquisition costs
Unproved	0.0	0.0	0.5
Proved	0.0	(20.4)	0.0
Total acquisition costs	0.0	(20.4)	0.5
Refunded exploration costs			(0.4)
Exploration costs	0.8	0.4
Development costs	208.5	102.4	120.8
Total costs incurred	209.3	82.4	120.9
Charged to expense
Dry hole expense	0.0	0.0
Geophysical and other costs	0.8	0.4	0.7
Total charged to expense	0.8	0.4	0.7
Property additions	208.5	82.0	120.2
Other
Property acquisition costs
Unproved	0.0	0.0	7.3
Proved	0.0	0.0	0.0
Total acquisition costs	0.0	0.0	7.3
Refunded exploration costs			24.7
Exploration costs	70.3	30.1
Development costs	4.3	3.7	6.8
Total costs incurred	74.6	33.8	38.8
Charged to expense
Dry hole expense	59.1	0.0
Geophysical and other costs	21.1	19.3	23.6
Total charged to expense	80.2	19.3	23.6
Property additions	$ (5.7)	$ 14.5	$ 15.2