Income Taxes - Components of the federal deferred tax asset (liability) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Long-term deferred tax asset (liability) [Abstract]
Prepaid and other insurance	$ (684)	$ (1,058)
Property	(2,497)	(7,341)
Investments in unconsolidated affiliates	623	606
Valuation allowance related to investments in unconsolidated affiliates	(623)	0
Uniform capitalization	0	729
Other	(121)	(93)
Net long-term deferred tax liability	(3,302)	(7,157)
Net deferred tax liability	$ (3,302)	$ (7,157)