Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 1,200		$ 1,200		$ 1,100
Operating Revenue	154,128	$ 160,873	306,530	$ 327,191
Revenue from Contract with Customer, Including Assessed Tax	2,800	3,400	5,900	6,900
fuel surcharge [Member]
Disaggregation of Revenue [Line Items]
Fuel surcharge revenue	$ 19,100	$ 14,000	$ 35,900	$ 33,400