Note 11 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating revenue	$ 251,280	$ 216,011	$ 706,768	$ 647,041
Operating Income	28,496	24,400	81,012	67,688
Truckload [Member]
Operating revenue	99,617	93,623	290,473	282,955
Operating Income	11,670	10,546	36,282	28,367
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	86,889	85,074	254,441	254,897
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	12,728	8,549	36,032	28,058
Dedicated [Member]
Operating revenue	82,162	78,337	240,520	228,801
Operating Income	8,521	11,024	28,074	31,009
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	68,826	69,002	202,955	200,237
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	13,336	9,335	37,565	28,564
Intermodal [Member]
Operating revenue	26,747	21,976	74,343	65,957
Operating Income	2,840	1,304	6,151	3,564
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	22,716	19,991	64,193	59,127
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	4,031	1,985	10,150	6,830
Brokerage [Member]
Operating revenue	42,754	22,075	101,432	69,328
Operating Income	$ 5,465	$ 1,526	$ 10,505	$ 4,748