Note 3 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenue	$ 197,024	$ 171,511	$ 383,984	$ 344,670
Operating income:
Operating Income	18,223	15,569	31,817	29,507
Truckload [Member]
Operating revenue	94,404	94,914	187,421	190,572
Operating income:
Operating Income	8,689	7,511	15,504	13,485
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	80,525	84,480	160,741	169,291
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	13,879	10,434	26,680	21,281
Dedicated [Member]
Operating revenue	56,971	41,502	105,804	81,779
Operating income:
Operating Income	5,584	5,074	8,072	9,561
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	47,232	38,601	89,596	75,500
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	9,739	2,901	16,208	6,279
Intermodal [Member]
Operating revenue	25,470	19,115	50,122	38,301
Operating income:
Operating Income	2,512	2,040	5,490	4,189
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	21,291	16,877	42,099	33,688
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	4,179	2,238	8,023	4,613
Brokerage [Member]
Operating revenue	20,179	15,980	40,637	34,018
Operating income:
Operating Income	$ 1,438	$ 944	$ 2,751	$ 2,272