Acquisitions and Divestitures Other Divestitures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment	$ 243.6	$ 806.8	$ 29.0
Net gain (loss) from asset sales, inclusive of restructuring costs	25.0	213.5	5.0
Other Property [Member]
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment	90.6
Net gain (loss) from asset sales, inclusive of restructuring costs	$ 38.5
Central Basin Platform [Member]
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment		3.5
Other Northern [Member]
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment		85.1
Net gain (loss) from asset sales, inclusive of restructuring costs		$ 33.1
Other Southern [Member]
Divestitures [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment			29.0
Net gain (loss) from asset sales, inclusive of restructuring costs			$ 8.6