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, 2020	Dec. 31, 2019	Dec. 31, 2018
Property acquisition costs
Unproved	$ 14.3	$ 547.0	$ 3.0
Proved	0.2	733.1	794.3
Total acquisition costs	14.5	1,280.1	797.3
Cost Incurred, Refunded Exploration Costs			126.0
Exploration costs	58.6	118.6
Development costs	736.8	1,282.4	1,390.0
Total costs incurred	809.9	2,681.1	2,313.3
Charged to expense
Dry hole expense			20.6
Geophysical and other costs	38.6	54.3	47.4
Total charged to expense	38.6	54.3	68.0
Property additions	771.3	2,626.8	2,245.3
U.S.
Property acquisition costs
Unproved	6.5	533.8	2.8
Proved	0.2	733.1	794.3
Total acquisition costs	6.7	1,266.9	797.1
Cost Incurred, Refunded Exploration Costs			88.1
Exploration costs	34.3	44.8
Development costs	609.2	979.0	853.7
Total costs incurred	650.2	2,290.7	1,738.9
Charged to expense
Dry hole expense			16.0
Geophysical and other costs	14.3	21.6	13.4
Total charged to expense	14.3	21.6	29.4
Property additions	635.9	2,269.1	1,709.5
Canada
Property acquisition costs
Unproved	0.5	0.2	0.0
Proved	0.0	0.0	0.0
Total acquisition costs	0.5	0.2	0.0
Cost Incurred, Refunded Exploration Costs			0.6
Exploration costs	(0.4)	6.4
Development costs	120.8	281.8	373.8
Total costs incurred	120.9	288.4	374.4
Charged to expense
Dry hole expense			0.0
Geophysical and other costs	0.7	0.5	0.6
Total charged to expense	0.7	0.5	0.6
Property additions	120.2	287.9	373.8
Malaysia
Property acquisition costs
Unproved	0.0	0.0	0.0
Proved	0.0	0.0	0.0
Total acquisition costs	0.0	0.0	0.0
Cost Incurred, Refunded Exploration Costs			2.2
Exploration costs	0.0	0.0
Development costs	0.0	0.0	145.9
Total costs incurred	0.0	0.0	148.1
Charged to expense
Dry hole expense			0.1
Geophysical and other costs	0.0	0.0	2.1
Total charged to expense	0.0	0.0	2.2
Property additions	0.0	0.0	145.9
Other
Property acquisition costs
Unproved	7.3	13.0	0.2
Proved	0.0	0.0	0.0
Total acquisition costs	7.3	13.0	0.2
Cost Incurred, Refunded Exploration Costs			35.1
Exploration costs	24.7	67.4
Development costs	6.8	21.6	16.6
Total costs incurred	38.8	102.0	51.9
Charged to expense
Dry hole expense			4.5
Geophysical and other costs	23.6	32.2	31.3
Total charged to expense	23.6	32.2	35.8
Property additions	15.2	69.8	16.1
Asset retirement obligations
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	14.7	79.6	373.5
Total costs incurred	14.7	79.6	373.5
Asset retirement obligations | U.S.
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	12.8	75.8	366.0
Total costs incurred	12.8	75.8	366.0
Asset retirement obligations | Canada
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	1.9	3.8	0.0
Total costs incurred	1.9	3.8	0.0
Asset retirement obligations | Malaysia
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	0.0	0.0	7.3
Total costs incurred	0.0	0.0	7.3
Asset retirement obligations | Other
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	0.0	0.0	0.2
Total costs incurred	$ 0.0	$ 0.0	$ 0.2