Note 12 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating revenue	$ 223,046	$ 218,646
Operating Income	23,998	18,032
Truckload [Member]
Operating revenue	94,915	95,132
Operating Income	11,415	6,785
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	83,919	83,857
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	10,996	11,275
Dedicated [Member]
Operating revenue	78,237	75,037
Operating Income	8,936	8,533
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	66,902	64,159
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	11,335	10,878
Intermodal [Member]
Operating revenue	22,004	23,680
Operating Income	1,461	1,306
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	19,446	20,594
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	2,558	3,086
Brokerage [Member]
Operating revenue	27,890	24,797
Operating Income	$ 2,186	$ 1,408