Note 12 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating revenue	$ 232,442	$ 212,384	$ 455,488	$ 431,030
Operating Income	28,518	25,256	52,516	43,288
Truckload [Member]
Operating revenue	95,941	94,200	190,856	189,332
Operating Income	13,197	11,036	24,612	17,821
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	83,633	85,966	167,552	169,823
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	12,308	8,234	23,304	19,509
Dedicated [Member]
Operating revenue	80,121	75,427	158,358	150,464
Operating Income	10,617	11,452	19,553	19,985
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	67,227	67,076	134,129	131,235
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	12,894	8,351	24,229	19,229
Intermodal [Member]
Operating revenue	25,592	20,301	47,596	43,981
Operating Income	1,850	954	3,311	2,260
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	22,031	18,542	41,477	39,136
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	3,561	1,759	6,119	4,845
Brokerage [Member]
Operating revenue	30,788	22,456	58,678	47,253
Operating Income	$ 2,854	$ 1,814	$ 5,040	$ 3,222