Revenue Recognition (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Contract with Customer, Asset, Net [Abstract]
Contract with Customer, Asset, Net	$ 1,100	$ 1,200
Operating Revenues	645,300	596,800	$ 610,800
Fuel surcharge revenue	61,700	75,000	85,300
Accessorial and other revenues	14,300	13,500	$ 14,900
Deposit Contracts, Liabilities	$ 0	$ 0