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, 2016	Dec. 31, 2015	Dec. 31, 2014
Property acquisition costs
Unproved		$ 18.6	$ 12.6	$ 92.9
Proved		206.7		7.4
Total acquisition costs		225.3	12.6	100.3
Exploration costs	[1]	70.1	371.9	430.1
Development costs	[1]	508.0	1,819.4	3,282.7
Total Costs		803.4	2,203.9	3,813.1
Charged to expense
Dry hole expense		15.1	296.8	270.0
Geophysical and other costs		43.4	98.7	169.2
Total charged to expense		58.5	395.5	439.2
Property additions		744.9	1,808.4	3,373.9
Exploration costs	[1]	70.1	371.9	430.1
Development costs	[1]	508.0	1,819.4	3,282.7
Total costs incurred		803.4	2,203.9	3,813.1
United States [Member]
Property acquisition costs
Unproved		18.6	10.1	92.9
Proved				7.4
Total acquisition costs		18.6	10.1	100.3
Exploration costs	[1]	18.5	166.8	160.0
Development costs	[1]	239.7	1,375.1	1,934.7
Total Costs		276.8	1,552.0	2,195.0
Charged to expense
Dry hole expense		0.4	241.3	92.1
Geophysical and other costs		5.7	16.9	37.7
Total charged to expense		6.1	258.2	129.8
Property additions		270.7	1,293.8	2,065.2
Exploration costs	[1]	18.5	166.8	160.0
Development costs	[1]	239.7	1,375.1	1,934.7
Total costs incurred		276.8	1,552.0	2,195.0
Canada [Member]
Property acquisition costs
Unproved			2.5
Proved		206.7
Total acquisition costs		206.7	2.5
Exploration costs	[1]	3.6	0.7	1.7
Development costs	[1]	165.1	231.5	413.8
Total Costs		375.4	234.7	415.5
Charged to expense
Geophysical and other costs		3.6	0.7	1.7
Total charged to expense		3.6	0.7	1.7
Property additions		371.8	234.0	413.8
Exploration costs	[1]	3.6	0.7	1.7
Development costs	[1]	165.1	231.5	413.8
Total costs incurred		375.4	234.7	415.5
Malaysia [Member]
Property acquisition costs
Exploration costs	[1]	6.0	69.0	6.3
Development costs	[1]	102.9	210.0	926.6
Total Costs		108.9	279.0	932.9
Charged to expense
Dry hole expense		4.5	29.7	47.4
Geophysical and other costs		0.7	7.9	1.3
Total charged to expense		5.2	37.6	48.7
Property additions		103.7	241.4	884.2
Exploration costs	[1]	6.0	69.0	6.3
Development costs	[1]	102.9	210.0	926.6
Total costs incurred		108.9	279.0	932.9
Other Regions [Member]
Property acquisition costs
Exploration costs	[1]	42.0	135.4	262.1
Development costs	[1]	0.3	2.8	7.6
Total Costs		42.3	138.2	269.7
Charged to expense
Dry hole expense		10.2	25.8	130.5
Geophysical and other costs		33.4	73.2	128.5
Total charged to expense		43.6	99.0	259.0
Property additions		(1.3)	39.2	10.7
Exploration costs	[1]	42.0	135.4	262.1
Development costs	[1]	0.3	2.8	7.6
Total costs incurred		42.3	138.2	269.7
Asset Retirement Obligation Costs [Member]
Property acquisition costs
Development costs		13.7	76.8	70.6
Total Costs		13.7	76.8	70.6
Charged to expense
Development costs		13.7	76.8	70.6
Total costs incurred		13.7	76.8	70.6
Asset Retirement Obligation Costs [Member] | United States [Member]
Property acquisition costs
Development costs		0.9	30.7	36.5
Total Costs		0.9	30.7	36.5
Charged to expense
Development costs		0.9	30.7	36.5
Total costs incurred		0.9	30.7	36.5
Asset Retirement Obligation Costs [Member] | Canada [Member]
Property acquisition costs
Development costs		10.5	49.1	(32.1)
Total Costs		10.5	49.1	(32.1)
Charged to expense
Development costs		10.5	49.1	(32.1)
Total costs incurred		10.5	49.1	(32.1)
Asset Retirement Obligation Costs [Member] | Malaysia [Member]
Property acquisition costs
Development costs		2.3	(3.0)	66.2
Total Costs		2.3	(3.0)	66.2
Charged to expense
Development costs		2.3	(3.0)	66.2
Total costs incurred		$ 2.3	$ (3.0)	$ 66.2

[1] Includes noncash asset retirement costs as follows: 2016 Exploration costs$ - - - - - Development costs 0.9 10.5 2.3 - 13.7 $ 0.9 10.5 2.3 - 13.7 2015 Exploration costs$ - - - - - Development costs 30.7 49.1 (3.0) - 76.8 $ 30.7 49.1 (3.0) - 76.8 2014 Exploration costs$ - - - - - Development costs 36.5 (32.1) 66.2 - 70.6 $ 36.5 (32.1) 66.2 - 70.6