Summary of Significant Accounting Policies - Components of other operating income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Operating Income [Abstract]
Property lease guaranty revenue	$ 726	$ 3,058	$ 3,890
Interest income	361	473	579
Management fee income	276	369	370
Other	284	249	208
Other operating	$ 1,647	$ 4,149	$ 5,047