Property and Equipment - Sales of Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Sales of used trucks and equipment	$ 594	$ 1,966	$ 535
Trucks [Member]
Property, Plant and Equipment [Line Items]
Sales of used trucks and equipment	$ 594	$ 1,966	$ 535