Impairment of Property and Equipment (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Impaired Long-Lived Assets Held and Used [Line Items]
Provisions for impairment of proved properties	$ 12.0	$ 89.8	$ 5.9
Provisions for impairment of unproved properties	15.4	3.4	1.4
Non Core Properties
Impaired Long-Lived Assets Held and Used [Line Items]
Provisions for impairment of proved properties	$ 12.0	$ 89.8	$ 5.9