Debt - Credit Facility - Wells Fargo Bank (Details) - Company Credit Facility [Member] $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) redetermindation	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 5,000,000
Number of interim redeterminations that may be requested | redetermindation	2
Period of redeterminations (in months)	12 months
Current borrowing base	$ 2,000,000
Elected borrowing base	1,000,000
Long-term Debt, Gross	321,500	$ 397,000
Remaining Borrowing Capacity	$ 678,500
Financial covenant, reduction of borrowing base (percentage)	25.00%
Minimum [Member]
Line of Credit Facility [Line Items]
Quarterly commitment fee percentage based on unused portion of borrowing base	0.375%
Maximum [Member]
Line of Credit Facility [Line Items]
Quarterly commitment fee percentage based on unused portion of borrowing base	0.50%
Federal Funds Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (percentage)	0.50%
LIBOR [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (percentage)	1.00%
LIBOR [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (percentage)	1.25%
LIBOR [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (percentage)	2.25%
Base Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (percentage)	0.25%
Base Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate (percentage)	1.25%