Capitalized Exploratory Well Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Capitalized Exploratory Well Costs [Abstract]
Beginning Balance	$ 2.6	$ 12.6	$ 2.6
Additions to capitalized exploratory well costs pending the determination of proved reserves	11.7	6.0	13.7
Reclassifications to proved properties after the determination of proved reserves	0.0	(16.0)	0.0
Capitalized exploratory well costs charged to expense	(0.1)	0.0	3.7
Ending Balance	$ 14.2	$ 2.6	$ 12.6