Derivative Instruments (Schedule of (Gain) Loss on Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	$ (20,449)	$ 67,714	$ 62,835	$ 97,310
Crude Oil
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	(20,915)	53,437	41,846	82,948
Natural Gas Liquids
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	0	6,564	(6)	4,799
Natural Gas
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	(1,600)	153	(3,670)	(2,892)
ExL Acquisition
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	1,215	10,600	30,214	16,430
Niobrara Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	265	(1,705)	(2,912)	(2,090)
Marcellus Shale Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	438	205	919	675
Utica Shale Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	$ 148	$ (1,540)	$ (3,556)	$ (2,560)