Capitalized Exploratory Well Costs Capitalized Exploratory Well Costs Table (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Capitalized Exploratory Well Costs [Line Items]
Capitalized exploratory well costs, beginning balance	$ 14.2
Additions to capitalized exploratory well costs	10.6
Capitalized exploratory well costs charged to expense	21.2	$ 0.0
Capitalized exploratory well costs, ending balance	0.0
Central Basin Platform [Member]
Capitalized Exploratory Well Costs [Line Items]
Reclassifications to proved properties	(3.6)
Capitalized exploratory well costs charged to expense	$ (21.2)