Note 12 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating revenue	$ 212,384	$ 212,090	$ 431,030	$ 411,113
Operating Income	25,256	19,944	43,288	37,677
Truckload [Member]
Operating revenue	94,200	96,022	189,332	187,744
Operating Income	11,036	8,045	17,821	15,600
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	85,966	83,215	169,823	163,382
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	8,234	12,807	19,509	24,362
Dedicated [Member]
Operating revenue	75,427	67,010	150,464	124,735
Operating Income	11,452	7,700	19,985	13,215
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	67,076	56,160	131,235	105,084
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	8,351	10,850	19,229	19,651
Intermodal [Member]
Operating revenue	20,301	20,603	43,981	43,574
Operating Income	954	1,536	2,260	3,930
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	18,542	17,527	39,136	37,282
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	1,759	3,076	4,845	6,292
Brokerage [Member]
Operating revenue	22,456	28,455	47,253	55,060
Operating Income	$ 1,814	$ 2,663	$ 3,222	$ 4,932