Capitalized Exploratory Well Costs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010
Capitalized exploratory well costs [Roll Forward]
Balance at January 1,	$ 5.0	$ 8.1	$ 5.0	$ 13.6
Additions to capitalized exploratory well costs pending the determination of proved reserves	0	0
Reclassifications to property, plant and equipment after the determination of proved reserves	0	(5.5)
Balance at March 31,	$ 5.0	$ 8.1	$ 5.0	$ 13.6