Note 13 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating revenue	$ 199,023	$ 186,960
Operating Income	17,733	13,594
Truckload [Member]
Operating revenue	91,722	93,017
Operating Income	7,555	6,815
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	80,167	80,216
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	11,555	12,801
Dedicated [Member]
Operating revenue	57,725	48,833
Operating Income	5,515	2,488
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	48,924	42,364
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	8,801	6,469
Intermodal [Member]
Operating revenue	22,971	24,652
Operating Income	2,394	2,978
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	19,755	20,808
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	3,216	3,844
Brokerage [Member]
Operating revenue	26,605	20,458
Operating Income	$ 2,269	$ 1,313