Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 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	152,612	$ 162,282	459,142	$ 489,473
Revenue from Contract with Customer, Including Assessed Tax	2,700	3,600	8,600	10,500
fuel surcharge [Member]
Disaggregation of Revenue [Line Items]
Fuel surcharge revenue	$ 19,600	$ 14,400	$ 55,500	$ 47,900