Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 1,000		$ 1,000		$ 1,100
Operating Revenue	142,144	$ 155,826	281,681	$ 312,521
Revenue from Contract with Customer, Including Assessed Tax	2,700	3,600	5,500	7,900
fuel surcharge [Member]
Disaggregation of Revenue [Line Items]
Fuel surcharge revenue	$ 18,100	$ 22,400	$ 35,100	$ 43,900