Capitalized Exploratory Well Costs (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Capitalized Exploratory Well Costs [Line Items]
Balance at January 1,		$ 0.0	$ 0.0	$ 14.2
Additions to capitalized exploratory well costs		0.0	0.0	10.7
Reclassifications to proved properties		0.0	0.0	(3.6)
Capitalized exploratory well costs charged to expense		0.0	0.0	(21.3)
Balance at December 31,	$ 0.0	0.0	0.0	0.0
Proceeds from Sale of Oil and Gas Property and Equipment		678.9	243.6	806.8
Impairment		$ 5.0	1,560.9	78.9
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