Note 14 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating revenue	$ 214,973	$ 199,649	$ 626,086	$ 583,633
Operating Income	20,034	18,993	57,711	50,810
Truckload [Member]
Operating revenue	94,987	93,920	282,731	281,341
Operating Income	6,956	10,026	22,556	25,530
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	82,931	80,563	246,313	241,304
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	12,056	13,357	36,418	40,037
Dedicated [Member]
Operating revenue	70,334	58,791	195,069	164,595
Operating Income	9,920	5,249	23,135	13,321
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	59,281	48,500	164,365	138,096
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	11,053	10,291	30,704	26,499
Intermodal [Member]
Operating revenue	22,297	25,939	65,871	76,061
Operating Income	1,210	2,507	5,140	7,997
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	19,336	21,735	56,618	63,834
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	2,961	4,204	9,253	12,227
Brokerage [Member]
Operating revenue	27,355	20,999	82,415	61,636
Operating Income	$ 1,948	$ 1,211	$ 6,880	$ 3,962