Capitalized Exploratory Well Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Capitalized Exploratory Well Costs [Abstract]
Beginning Balance	$ 12.6	$ 2.6	$ 2.1
Additions to capitalized exploratory well costs pending the determination of proved reserves	6.0	13.7	2.7
Reclassifications to proved properties after the determination of proved reserves	(16.0)	0.0	(2.2)
Capitalized exploratory well costs charged to expense	0.0	(3.7)	0.0
Ending Balance	$ 2.6	$ 12.6	$ 2.6