Income Taxes - Components of the federal deferred tax asset (liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Long-term deferred tax asset (liability) [Abstract]
Prepaid and other insurance	$ (170)	$ (684)
Property	(5,259)	(2,497)
Investments in unconsolidated affiliates	525	623
Valuation allowance related to investments in unconsolidated affiliates	(525)	(623)
Net operating loss	(1,436)	0
Other	(245)	(121)
Net long-term deferred tax liability	(4,238)	(3,302)
Net deferred tax liability	$ (4,238)	$ (3,302)