Capitalized Exploratory Well Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capitalized Exploratory Well Costs [Line Items]
Balance at January 1,	$ 0.0	$ 14.2	$ 2.6
Additions to capitalized exploratory well costs	0.0	10.7	11.7
Reclassifications to proved properties	0.0	(3.6)	0.0
Capitalized exploratory well costs charged to expense	0.0	(21.3)	0.1
Balance at December 31,	0.0	0.0	14.2
Proceeds from Sale of Oil and Gas Property and Equipment	243.6	806.8	29.0
Impairment	1,560.9	$ 78.9	$ 1,194.3
Central Basin Platform [Member]
Capitalized Exploratory Well Costs [Line Items]
Reclassifications to proved properties	(3.6)
Proceeds from Sale of Oil and Gas Property and Equipment	3.5
Impairment	$ 28.3