Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 1,200		$ 1,200		$ 1,100
Operating Revenue	147,908	$ 151,279	429,589	$ 463,800
Revenue from Contract with Customer, Including Assessed Tax	4,000	3,200	9,600	11,100
fuel surcharge [Member]
Disaggregation of Revenue [Line Items]
Fuel surcharge revenue	$ 18,400	$ 21,400	$ 53,500	$ 65,300