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, 2021	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2021
Property acquisition costs
Unproved	$ 8.8	$ 14.3	$ 547.0
Proved	(0.5)	0.2	733.1
Total acquisition costs	8.3	14.5	1,280.1
Refunded exploration costs			118.6
Exploration costs	62.2	58.6
Development costs	619.3	736.8	1,282.4
Total costs incurred	689.8	809.9	2,681.1
Charged to expense
Dry hole expense	17.3
Geophysical and other costs	32.8	38.6	54.3
Total charged to expense	50.1	38.6	54.3
Property additions	$ 639.7	771.3	2,626.8
Terra Nova
Charged to expense
Additional percentage acquired through commercial settlement	7.525%			7.525%
U.S.
Property acquisition costs
Unproved	$ 8.8	6.5	533.8
Proved	19.9	0.2	733.1
Total acquisition costs	28.7	6.7	1,266.9
Refunded exploration costs			44.8
Exploration costs	31.7	34.3
Development costs	513.2	609.2	979.0
Total costs incurred	573.6	650.2	2,290.7
Charged to expense
Dry hole expense	17.3
Geophysical and other costs	13.1	14.3	21.6
Total charged to expense	30.4	14.3	21.6
Property additions	543.2	635.9	2,269.1
Canada
Property acquisition costs
Unproved	0.0	0.5	0.2
Proved	(20.4)	0.0	0.0
Total acquisition costs	(20.4)	0.5	0.2
Refunded exploration costs			6.4
Exploration costs	0.4	(0.4)
Development costs	102.4	120.8	281.8
Total costs incurred	82.4	120.9	288.4
Charged to expense
Dry hole expense	0.0
Geophysical and other costs	0.4	0.7	0.5
Total charged to expense	0.4	0.7	0.5
Property additions	82.0	120.2	287.9
Other
Property acquisition costs
Unproved	0.0	7.3	13.0
Proved	0.0	0.0	0.0
Total acquisition costs	0.0	7.3	13.0
Refunded exploration costs			67.4
Exploration costs	30.1	24.7
Development costs	3.7	6.8	21.6
Total costs incurred	33.8	38.8	102.0
Charged to expense
Dry hole expense	0.0
Geophysical and other costs	19.3	23.6	32.2
Total charged to expense	19.3	23.6	32.2
Property additions	14.5	15.2	69.8
Asset retirement obligations
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	54.4	14.7	79.6
Total costs incurred	54.4	14.7	79.6
Asset retirement obligations | U.S.
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	23.7	12.8	75.8
Total costs incurred	23.7	12.8	75.8
Asset retirement obligations | Canada
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	29.3	1.9	3.8
Total costs incurred	29.3	1.9	3.8
Asset retirement obligations | Other
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	1.4	0.0	0.0
Total costs incurred	$ 1.4	$ 0.0	$ 0.0