Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Operating Revenue	$ 151,279	$ 182,114	$ 463,800	$ 441,632
Revenue from Contract with Customer, Including Assessed Tax	3,200	10,700	11,100	18,500
fuel surcharge [Member]
Disaggregation of Revenue [Line Items]
Fuel surcharge revenue	$ 21,400	$ 21,100	$ 65,300	$ 50,700