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, 2017	Dec. 31, 2016	Dec. 31, 2015
Property acquisition costs
Unproved	$ 63.4	$ 18.6	$ 12.6
Proved	7.7	206.7
Total acquisition costs	71.1	225.3	12.6
Exploration costs	79.2	70.1	371.9
Development costs	819.0	508.0	1,819.4
Total costs incurred	969.3	803.4	2,203.9
Charged to expense
Dry hole expense	(4.2)	15.1	296.8
Geophysical and other costs	65.2	43.4	98.7
Total charged to expense	61.0	58.5	395.5
Property additions	908.3	744.9	1,808.4
United States [Member]
Property acquisition costs
Unproved	50.4	18.6	10.1
Proved	7.7
Total acquisition costs	58.1	18.6	10.1
Exploration costs	13.7	18.5	166.8
Development costs	508.4	239.7	1,375.1
Total costs incurred	580.2	276.8	1,552.0
Charged to expense
Dry hole expense	(1.9)	0.4	241.3
Geophysical and other costs	9.7	5.7	16.9
Total charged to expense	7.8	6.1	258.2
Property additions	572.4	270.7	1,293.8
Canada [Member]
Property acquisition costs
Unproved			2.5
Proved		206.7
Total acquisition costs		206.7	2.5
Exploration costs	0.6	3.6	0.7
Development costs	273.8	165.1	231.5
Total costs incurred	274.4	375.4	234.7
Charged to expense
Geophysical and other costs	0.5	3.6	0.7
Total charged to expense	0.5	3.6	0.7
Property additions	273.9	371.8	234.0
Malaysia [Member]
Property acquisition costs
Exploration costs	(8.9)	6.0	69.0
Development costs	35.7	102.9	210.0
Total costs incurred	26.8	108.9	279.0
Charged to expense
Dry hole expense	0.7	4.5	29.7
Geophysical and other costs	1.7	0.7	7.9
Total charged to expense	2.4	5.2	37.6
Property additions	24.4	103.7	241.4
Other Regions [Member]
Property acquisition costs
Unproved	13.0
Total acquisition costs	13.0
Exploration costs	73.8	42.0	135.4
Development costs	1.1	0.3	2.8
Total costs incurred	87.9	42.3	138.2
Charged to expense
Dry hole expense	(3.0)	10.2	25.8
Geophysical and other costs	53.3	33.4	73.2
Total charged to expense	50.3	43.6	99.0
Property additions	37.6	(1.3)	39.2
Asset Retirement Obligation Costs [Member]
Property acquisition costs
Development costs	52.3	13.7	76.8
Total costs incurred	52.3	13.7	76.8
Asset Retirement Obligation Costs [Member] | United States [Member]
Property acquisition costs
Development costs	37.6	0.9	30.7
Total costs incurred	37.6	0.9	30.7
Asset Retirement Obligation Costs [Member] | Canada [Member]
Property acquisition costs
Development costs	6.3	10.5	49.1
Total costs incurred	6.3	10.5	49.1
Asset Retirement Obligation Costs [Member] | Malaysia [Member]
Property acquisition costs
Development costs	8.4	2.3	(3.0)
Total costs incurred	$ 8.4	$ 2.3	$ (3.0)