Note 14 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating revenue	$ 212,090	$ 197,024	$ 411,113	$ 383,984
Operating Income	19,944	18,223	37,677	31,817
Truckload [Member]
Operating revenue	96,022	94,404	187,744	187,421
Operating Income	8,045	8,689	15,600	15,504
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	83,215	80,525	163,382	160,741
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	12,807	13,879	24,362	26,680
Dedicated [Member]
Operating revenue	67,010	56,971	124,735	105,804
Operating Income	7,700	5,584	13,215	8,072
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	56,160	47,232	105,084	89,596
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	10,850	9,739	19,651	16,208
Intermodal [Member]
Operating revenue	20,603	25,470	43,574	50,122
Operating Income	1,536	2,512	3,930	5,490
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	17,527	21,291	37,282	42,099
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	3,076	4,179	6,292	8,023
Brokerage [Member]
Operating revenue	28,455	20,179	55,060	40,637
Operating Income	$ 2,663	$ 1,438	$ 4,932	$ 2,751