Supplemental Oil and Gas Information - Costs Incurred in Oil and Gas Property Acquisition, Exploration and Development Activities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property acquisition costs
Unproved	$ 547.0	$ 3.0	$ 63.4
Proved	733.1	794.3	7.7
Total acquisition costs	1,280.1	797.3	71.1
Cost Incurred, Refunded Exploration Costs			79.2
Exploration costs	118.6	126.0
Development costs	1,282.4	1,390.0	819.0
Total costs incurred	2,681.1	2,313.3	969.3
Charged to expense
Dry hole expense		20.6	(4.2)
Geophysical and other costs	54.3	47.4	65.2
Total charged to expense	54.3	68.0	61.0
Property additions	2,626.8	2,245.3	908.3
United States
Property acquisition costs
Unproved	533.8	2.8	50.4
Proved	733.1	794.3	7.7
Total acquisition costs	1,266.9	797.1	58.1
Cost Incurred, Refunded Exploration Costs			13.7
Exploration costs	44.8	88.1
Development costs	979.0	853.7	508.4
Total costs incurred	2,290.7	1,738.9	580.2
Charged to expense
Dry hole expense		16.0	(1.9)
Geophysical and other costs	21.6	13.4	9.7
Total charged to expense	21.6	29.4	7.8
Property additions	2,269.1	1,709.5	572.4
Canada
Property acquisition costs
Unproved	0.2	0.0	0.0
Proved	0.0	0.0	0.0
Total acquisition costs	0.2	0.0	0.0
Cost Incurred, Refunded Exploration Costs			0.6
Exploration costs	6.4	0.6
Development costs	281.8	373.8	273.8
Total costs incurred	288.4	374.4	274.4
Charged to expense
Dry hole expense		0.0	0.0
Geophysical and other costs	0.5	0.6	0.5
Total charged to expense	0.5	0.6	0.5
Property additions	287.9	373.8	273.9
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			(8.9)
Exploration costs	0.0	2.2
Development costs	0.0	145.9	35.7
Total costs incurred	0.0	148.1	26.8
Charged to expense
Dry hole expense		0.1	0.7
Geophysical and other costs	0.0	2.1	1.7
Total charged to expense	0.0	2.2	2.4
Property additions	0.0	145.9	24.4
Other
Property acquisition costs
Unproved	13.0	0.2	13.0
Proved	0.0	0.0	0.0
Total acquisition costs	13.0	0.2	13.0
Cost Incurred, Refunded Exploration Costs			73.8
Exploration costs	67.4	35.1
Development costs	21.6	16.6	1.1
Total costs incurred	102.0	51.9	87.9
Charged to expense
Dry hole expense		4.5	(3.0)
Geophysical and other costs	32.2	31.3	53.3
Total charged to expense	32.2	35.8	50.3
Property additions	69.8	16.1	37.6
Asset retirement obligations
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	79.6	373.5	52.3
Total costs incurred	79.6	373.5	52.3
Asset retirement obligations | United States
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	75.8	366.0	37.6
Total costs incurred	75.8	366.0	37.6
Asset retirement obligations | Canada
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	3.8	0.0	6.3
Total costs incurred	3.8	0.0	6.3
Asset retirement obligations | Malaysia
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	0.0	7.3	8.4
Total costs incurred	0.0	7.3	8.4
Asset retirement obligations | Other
Property acquisition costs
Exploration costs	0.0	0.0	0.0
Development costs	0.0	0.2	0.0
Total costs incurred	$ 0.0	$ 0.2	$ 0.0