Derivative Instruments (Schedule of (Gain) Loss on Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Loss on derivatives, net	$ 83,284	$ 29,596
Crude Oil
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Loss on derivatives, net	62,761	29,511
Natural Gas Liquids
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Loss on derivatives, net	(6)	(1,765)
Natural Gas
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Loss on derivatives, net	(2,070)	(3,045)
ExL Acquisition
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	28,999	5,830
Niobrara Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	(3,177)	(385)
Marcellus Shale Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	481	470
Utica Shale Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	$ (3,704)	$ (1,020)