Derivative Instruments (Schedule of Contingent Consideration) (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) $ / bbls $ / MMBTU	Mar. 31, 2018 USD ($)
Embedded Derivative [Line Items]
Divestiture date fair value of contingent consideration arrangement	$ 0	$ (7,880,000)
Cash paid for settlements of contingent consideration arrangements, net	(40,000,000)	0
Receipt For Contingent Consideration Asset, Financing Activities	10,000,000
ExL Acquisition
Embedded Derivative [Line Items]
Contingent Consideration, Liability, Acquisition Date Fair Value	(52,300,000)
Cash paid for settlements of contingent consideration arrangements, net	(50,000,000)	0
Remaining Potential Contingent Consideration Payments	$ (75,000,000)
ExL Acquisition | FY 2018 [Member]
Embedded Derivative [Line Items]
Threshold Price per Bbl for Additional Payments for Acquisition | $ / bbls	50.00
ExL Acquisition | FY 2019 [Member]
Embedded Derivative [Line Items]
Threshold Price per Bbl for Additional Payments for Acquisition | $ / bbls	50.00
Potential Additional Annual Payments for Acquisition	$ (50,000,000)
Niobrara Divestiture
Embedded Derivative [Line Items]
Divestiture date fair value of contingent consideration arrangement		7,880,000
Receipt For Contingent Consideration Asset, Financing Activities	5,000,000	0
Remaining Potential Contingent Consideration Receipts	$ 10,000,000
Niobrara Divestiture | FY 2018 [Member]
Embedded Derivative [Line Items]
Threshold Price Per Bbl For Additional Proceeds From Divestiture | $ / bbls	55.00
Niobrara Divestiture | FY 2019 [Member]
Embedded Derivative [Line Items]
Threshold Price Per Bbl For Additional Proceeds From Divestiture | $ / bbls	55.00
Potential Additional Annual Proceeds From Divestiture	$ 5,000,000
Niobrara Divestiture | FY 2020 [Member]
Embedded Derivative [Line Items]
Threshold Price Per Bbl For Additional Proceeds From Divestiture | $ / bbls	60.00
Potential Additional Annual Proceeds From Divestiture	$ 5,000,000
Marcellus Shale Divestiture
Embedded Derivative [Line Items]
Divestiture date fair value of contingent consideration arrangement		2,660,000
Receipt For Contingent Consideration Asset, Financing Activities	0
Remaining Potential Contingent Consideration Receipts	$ 6,000,000
Marcellus Shale Divestiture | FY 2018 [Member]
Embedded Derivative [Line Items]
Threshold Price per MMBtu for Additional Payments from Divestiture | $ / MMBTU	3.13
Marcellus Shale Divestiture | FY 2019 [Member]
Embedded Derivative [Line Items]
Threshold Price per MMBtu for Additional Payments from Divestiture | $ / MMBTU	3.18
Potential Additional Annual Proceeds From Divestiture	$ 3,000,000
Marcellus Shale Divestiture | FY 2020 [Member]
Embedded Derivative [Line Items]
Threshold Price per MMBtu for Additional Payments from Divestiture | $ / MMBTU	3.30
Potential Additional Annual Proceeds From Divestiture	$ 3,000,000
Utica Shale Divestiture
Embedded Derivative [Line Items]
Divestiture date fair value of contingent consideration arrangement		6,145,000
Receipt For Contingent Consideration Asset, Financing Activities	5,000,000	$ 0
Remaining Potential Contingent Consideration Receipts	$ 10,000,000
Utica Shale Divestiture | FY 2018 [Member]
Embedded Derivative [Line Items]
Threshold Price Per Bbl For Additional Proceeds From Divestiture | $ / bbls	50.00
Utica Shale Divestiture | FY 2019 [Member]
Embedded Derivative [Line Items]
Threshold Price Per Bbl For Additional Proceeds From Divestiture | $ / bbls	53.00
Potential Additional Annual Proceeds From Divestiture	$ 5,000,000
Utica Shale Divestiture | FY 2020 [Member]
Embedded Derivative [Line Items]
Threshold Price Per Bbl For Additional Proceeds From Divestiture | $ / bbls	56.00
Potential Additional Annual Proceeds From Divestiture	$ 5,000,000