Note 12 - Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating revenue	$ 171,511	$ 166,090	$ 344,670	$ 328,019
Operating Income	15,569	14,776	29,507	28,901
Truckload [Member]
Operating revenue	94,914	94,036	190,572	184,090
Operating Income	7,511	6,951	13,485	13,891
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	84,480	85,103	169,291	168,045
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	10,434	8,933	21,281	16,045
Dedicated [Member]
Operating revenue	41,502	39,275	81,779	76,372
Operating Income	5,074	5,134	9,561	9,458
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	38,601	36,654	75,500	72,164
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	2,901	2,621	6,279	4,208
Intermodal [Member]
Operating revenue	19,115	17,782	38,301	34,983
Operating Income	2,040	1,822	4,189	3,751
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue	16,877	16,118	33,688	31,972
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue	2,238	1,664	4,613	3,011
Brokerage [Member]
Operating revenue	15,980	14,997	34,018	32,574
Operating Income	$ 944	$ 869	$ 2,272	$ 1,801