Income Taxes - Income Tax Temporary Differences Table (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Depreciable property	$ (7,676)	$ (4,976)
Credit carryforwards	448	498
Other intangible assets	(189)	(1,928)
Other	8	4
Net long-term deferred income tax liabilities	$ (7,409)	$ (6,402)