Derivative Instruments (Schedule of (Gain) Loss on Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	$ 67,714	$ (26,065)	$ 97,310	$ (51,381)
Crude Oil
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	53,437	(29,736)	82,948	(48,163)
Natural Gas Liquids
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	6,564	0	4,799	0
Natural Gas
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	153	(3,883)	(2,892)	(10,719)
Deferred Premiums on Derivative Instruments
Derivative Instruments, Gain (Loss), Commodity [Line Items]
(Gain) loss on derivatives, net	0	7,554	0	7,501
ExL Acquisition
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	10,600	0	16,430	0
Niobrara Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	(1,705)	0	(2,090)	0
Marcellus Shale Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	205	0	675	0
Utica Shale Divestiture
Derivative Instruments, Gain (Loss), Commodity [Line Items]
Gain (Loss) on Embedded Derivative, Net	$ (1,540)	$ 0	$ (2,560)	$ 0