Note 13 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating revenue	$ 216,011	$ 214,973	$ 647,041	$ 626,086
Operating Income	24,400	20,034	67,688	57,711
Truckload [Member]
Operating revenue	93,623	94,987	282,955	282,731
Operating Income	10,546	6,956	28,367	22,556
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	85,074	82,931	254,897	246,313
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	8,549	12,056	28,058	36,418
Dedicated [Member]
Operating revenue	78,337	70,334	228,801	195,069
Operating Income	11,024	9,920	31,009	23,135
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	69,002	59,281	200,237	164,365
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	9,335	11,053	28,564	30,704
Intermodal [Member]
Operating revenue	21,976	22,297	65,957	65,871
Operating Income	1,304	1,210	3,564	5,140
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	19,991	19,336	59,127	56,618
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	1,985	2,961	6,830	9,253
Brokerage [Member]
Operating revenue	22,075	27,355	69,328	82,415
Operating Income	$ 1,526	$ 1,948	$ 4,748	$ 6,880