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, 2018	Dec. 31, 2017	Dec. 31, 2016
Property acquisition costs
Unproved	$ 3.0	$ 63.4	$ 18.6
Proved	794.3	7.7	206.7
Total acquisition costs	797.3	71.1	225.3
Exploration costs	126.0	79.2	70.1
Development costs	1,390.0	819.0	508.0
Total costs incurred	2,313.3	969.3	803.4
Charged to expense
Dry hole expense	20.6	(4.2)	15.1
Geophysical and other costs	47.4	65.2	43.4
Total charged to expense	68.0	61.0	58.5
Property additions	2,245.3	908.3	744.9
United States [Member]
Property acquisition costs
Unproved	2.8	50.4	18.6
Proved	794.3	7.7
Total acquisition costs	797.1	58.1	18.6
Exploration costs	88.1	13.7	18.5
Development costs	853.7	508.4	239.7
Total costs incurred	1,738.9	580.2	276.8
Charged to expense
Dry hole expense	16.0	(1.9)	0.4
Geophysical and other costs	13.4	9.7	5.7
Total charged to expense	29.4	7.8	6.1
Property additions	1,709.5	572.4	270.7
Canada [Member]
Property acquisition costs
Proved			206.7
Total acquisition costs			206.7
Exploration costs	0.6	0.6	3.6
Development costs	373.8	273.8	165.1
Total costs incurred	374.4	274.4	375.4
Charged to expense
Geophysical and other costs	0.6	0.5	3.6
Total charged to expense	0.6	0.5	3.6
Property additions	373.8	273.9	371.8
Malaysia [Member]
Property acquisition costs
Exploration costs	2.2	(8.9)	6.0
Development costs	145.9	35.7	102.9
Total costs incurred	148.1	26.8	108.9
Charged to expense
Dry hole expense	0.1	0.7	4.5
Geophysical and other costs	2.1	1.7	0.7
Total charged to expense	2.2	2.4	5.2
Property additions	145.9	24.4	103.7
Other Regions [Member]
Property acquisition costs
Unproved	0.2	13.0
Total acquisition costs	0.2	13.0
Exploration costs	35.1	73.8	42.0
Development costs	16.6	1.1	0.3
Total costs incurred	51.9	87.9	42.3
Charged to expense
Dry hole expense	4.5	(3.0)	10.2
Geophysical and other costs	31.3	53.3	33.4
Total charged to expense	35.8	50.3	43.6
Property additions	16.1	37.6	(1.3)
Asset Retirement Obligation Costs [Member]
Property acquisition costs
Development costs	373.5	52.3	13.7
Total costs incurred	373.5	52.3	13.7
Asset Retirement Obligation Costs [Member] | United States [Member]
Property acquisition costs
Development costs	366.0	37.6	0.9
Total costs incurred	366.0	37.6	0.9
Asset Retirement Obligation Costs [Member] | Canada [Member]
Property acquisition costs
Development costs		6.3	10.5
Total costs incurred		6.3	10.5
Asset Retirement Obligation Costs [Member] | Malaysia [Member]
Property acquisition costs
Development costs	7.3	8.4	2.3
Total costs incurred	7.3	$ 8.4	$ 2.3
Asset Retirement Obligation Costs [Member] | Other Regions [Member]
Property acquisition costs
Development costs	0.2
Total costs incurred	$ 0.2