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Restructuring Charges (Summary of Restructuring Charges) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Term of contract					10 years
Total restructuring charges	$ 4.0	$ 2.0	$ 9.0	$ 20.0
E&P
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	4.0	2.0	9.0	18.0
Midstream
Restructuring Cost and Reserve [Line Items]
Total restructuring charges				2.0
Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring liability	1.0		1.0			$ 5.0
Severance (including payroll taxes)	1.0	1.0	4.0	18.0
Office consolidation	3.0	0.0	5.0	0.0
Professional fees	0.0	1.0	0.0	2.0
Total restructuring charges	4.0	2.0	9.0	20.0
Workforce Reduction \| E&P
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 4.0	$ 2.0	$ 9.0	18.0
Workforce Reduction \| Midstream
Restructuring Cost and Reserve [Line Items]
Total restructuring charges				$ 2.0