Impairment of Property and Equipment (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Impaired Long-Lived Assets Held and Used [Line Items]
Provisions for impairment of proved properties	$ 5.9	$ 10.4	$ 11.9
Provisions for impairment of unproved properties	1.4	6.2	7.8
Non Core Properties [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Provisions for impairment of proved properties	$ 5.4	$ 10.4	$ 11.1