Derivative Instruments (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative [Line Items]
Payment for Contingent Consideration Liability, Financing Activities	$ 40,000	$ 0
Receipt For Contingent Consideration Asset, Financing Activities	10,000
ExL Acquisition
Derivative [Line Items]
Potential Future Payments For Contingent Consideration Liability Financing Activities	(2,300)
Payment for Contingent Consideration Liability, Financing Activities	50,000	0
Niobrara Divestiture
Derivative [Line Items]
Potential Future Cash Receipts For Contingent Consideration Asset Financing Activities	2,900
Receipt For Contingent Consideration Asset, Financing Activities	5,000	0
Marcellus Shale Divestiture
Derivative [Line Items]
Potential Future Cash Receipts For Contingent Consideration Asset Financing Activities	2,700
Receipt For Contingent Consideration Asset, Financing Activities	0
Utica Shale Divestiture
Derivative [Line Items]
Potential Future Cash Receipts For Contingent Consideration Asset Financing Activities	1,100
Receipt For Contingent Consideration Asset, Financing Activities	$ 5,000	$ 0