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, 2015	Dec. 31, 2014		Dec. 31, 2013
Property acquisition costs
Unproved		$ 12.6	$ 92.9		$ 35.6
Proved			7.4		13.2
Total acquisition costs		12.6	100.3		48.8
Exploration costs	[1]	371.9	430.1		493.7
Development costs	[1]	1,819.4	3,282.7		3,939.6
Total Costs		2,203.9	3,813.1		4,482.1
Charged to expense
Dry hole expense		296.8	270.0		262.9
Geophysical and other costs		98.7	169.2		172.4
Total charged to expense		395.5	439.2		435.3
Property additions		1,808.4	3,373.9		4,046.8
Exploration costs	[1]	371.9	430.1		493.7
Development costs	[1]	1,819.4	3,282.7		3,939.6
Total costs incurred		2,203.9	3,813.1		4,482.1
United States [Member]
Property acquisition costs
Unproved		10.1	92.9		32.4
Proved			7.4		13.2
Total acquisition costs		10.1	100.3		45.6
Exploration costs	[1]	166.8	160.0		112.4
Development costs	[1]	1,375.1	1,934.7		1,773.2
Total Costs		1,552.0	2,195.0		1,931.2
Charged to expense
Dry hole expense		241.3	92.1		46.1
Geophysical and other costs		16.9	37.7		29.1
Total charged to expense		258.2	129.8		75.2
Property additions		1,293.8	2,065.2		1,856.0
Exploration costs	[1]	166.8	160.0		112.4
Development costs	[1]	1,375.1	1,934.7		1,773.2
Total costs incurred		1,552.0	2,195.0		1,931.2
Canada [Member]
Property acquisition costs
Unproved		2.5
Total acquisition costs		2.5
Exploration costs	[1]	0.7	1.7		21.8
Development costs	[1]	231.5	413.8		351.6
Total Costs		234.7	415.5		373.4
Charged to expense
Dry hole expense					32.1
Geophysical and other costs		0.7	1.7		0.7
Total charged to expense		0.7	1.7		32.8
Property additions		234.0	413.8		340.6
Exploration costs	[1]	0.7	1.7		21.8
Development costs	[1]	231.5	413.8		351.6
Total costs incurred		234.7	415.5		373.4
Malaysia [Member]
Property acquisition costs
Exploration costs	[1]	69.0	6.3		14.9
Development costs	[1]	210.0	926.6	[2]	1,787.7	[2]
Total Costs		279.0	932.9		1,802.6
Charged to expense
Dry hole expense		29.7	47.4		20.7
Geophysical and other costs		7.9	1.3		4.6
Total charged to expense		37.6	48.7		25.3
Property additions		241.4	884.2		1,777.3
Exploration costs	[1]	69.0	6.3		14.9
Development costs	[1]	210.0	926.6	[2]	1,787.7	[2]
Total costs incurred		279.0	932.9		1,802.6
Non-cash capital lease obligation					358.0
United Kingdom [Member]
Property acquisition costs
Development costs	[1],[3]				8.1
Total Costs	[3]				8.1
Charged to expense
Property additions	[3]				8.1
Development costs	[1],[3]				8.1
Total costs incurred	[3]				8.1
Other Regions [Member]
Property acquisition costs
Unproved					3.2
Total acquisition costs					3.2
Exploration costs	[1]	135.4	262.1		344.6
Development costs	[1]	2.8	7.6		19.0
Total Costs		138.2	269.7		366.8
Charged to expense
Dry hole expense		25.8	130.5		164.0
Geophysical and other costs		73.2	128.5		138.0
Total charged to expense		99.0	259.0		302.0
Property additions		39.2	10.7		64.8
Exploration costs	[1]	135.4	262.1		344.6
Development costs	[1]	2.8	7.6		19.0
Total costs incurred		138.2	269.7		366.8
Asset Retirement Obligation Costs [Member]
Property acquisition costs
Exploration costs					0.2
Development costs		76.8	70.6		171.9
Total Costs		76.8	70.6		172.1
Charged to expense
Exploration costs					0.2
Development costs		76.8	70.6		171.9
Total costs incurred		76.8	70.6		172.1
Asset Retirement Obligation Costs [Member] | United States [Member]
Property acquisition costs
Development costs		30.7	36.5		70.1
Total Costs		30.7	36.5		70.1
Charged to expense
Development costs		30.7	36.5		70.1
Total costs incurred		30.7	36.5		70.1
Asset Retirement Obligation Costs [Member] | Canada [Member]
Property acquisition costs
Exploration costs					0.2
Development costs		49.1	(32.1)		5.9
Total Costs		49.1	(32.1)		6.1
Charged to expense
Exploration costs					0.2
Development costs		49.1	(32.1)		5.9
Total costs incurred		49.1	(32.1)		6.1
Asset Retirement Obligation Costs [Member] | Malaysia [Member]
Property acquisition costs
Development costs		(3.0)	66.2		95.9
Total Costs		(3.0)	66.2		95.9
Charged to expense
Development costs		(3.0)	66.2		95.9
Total costs incurred		$ (3.0)	$ 66.2		$ 95.9

[1] Includes non-cash asset retirement costs as follows:

[2] Includes property costs associated with non-cash capital lease of $358.0 million at the Kakap field.

[3] The Company has accounted for U.K. operations as discontinued operations due to the sale of these operations in the first half of 2013.