Reduction in Workforce (Summary of Restructuring Charges) (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 18,000,000	$ 18,000,000
E&P [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	16,000,000	16,000,000
Midstream [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	2,000,000	2,000,000
Workforce Reduction [Member]
Restructuring Cost and Reserve [Line Items]
Severance (including payroll taxes)	17,000,000	17,000,000
Professional fees	1,000,000	1,000,000
Total restructuring charges	18,000,000	18,000,000
Workforce Reduction [Member] | Unvested Stock-Based Awards [Member]
Restructuring Cost and Reserve [Line Items]
Severance (including payroll taxes)	2,000,000	2,000,000
Workforce Reduction [Member] | Non-Cash Stock-Based Awards [Member]
Restructuring Cost and Reserve [Line Items]
Severance (including payroll taxes)	358,000	358,000
Workforce Reduction [Member] | E&P [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	16,000,000	16,000,000
Workforce Reduction [Member] | Midstream [Member]
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 2,000,000	$ 2,000,000