Capitalized Exploratory Well Costs (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net changes in capitalized exploratory well costs [Roll Forward]
Balance at beginning of period	$ 13.6	$ 51.7	$ 17.0
Additions to capitalized exploratory well costs pending the determination of proved reserves	0	12.2	51.7
Reclassifications to property, plant and equipment after the determination of proved reserves	(8.3)	(50.3)	(14.3)
Capitalized exploratory well costs charged to expense	(0.3)	0	(2.7)
Balance at end of period	$ 5.0	$ 13.6	$ 51.7