Note 3 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating revenue	$ 186,960	$ 173,159
Operating income:
Operating Income	13,594	13,938
Truckload [Member]
Operating revenue	93,017	95,658
Operating income:
Operating Income	6,815	5,974
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	80,216	84,811
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	12,801	10,847
Dedicated [Member]
Operating revenue	48,833	40,277
Operating income:
Operating Income	2,488	4,487
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	42,364	36,899
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	6,469	3,378
Intermodal [Member]
Operating revenue	24,652	19,186
Operating income:
Operating Income	2,978	2,149
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	20,808	16,811
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	3,844	2,375
Brokerage [Member]
Operating revenue	20,458	18,038
Operating income:
Operating Income	$ 1,313	$ 1,328