Income Taxes (Schedule of Net deferred tax) (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Deferred tax assets:
Net operating loss carryforward	$ 19,086
Asset retirement obligation	26,948
Other	2,365
Total deferred tax assets before valuation allowance	48,399
Valuation allowance	(15,573)
Net deferred tax assets	32,826
Deferred tax liability:
Oil and natural gas properties	30,200
Derivative instruments	2,626
Total deferred tax liability	32,826
Total net deferred tax