Note 3 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenue	$ 203,961	$ 199,649	$ 197,024	$ 186,960	$ 182,771	$ 170,679	$ 171,511	$ 173,159	$ 787,594	$ 698,120	$ 671,144
Operating income:
Operating income	$ 19,538	$ 18,993	$ 18,223	$ 13,594	$ 14,333	$ 13,022	$ 15,569	$ 13,938	70,348	56,862	58,303
Truckload [Member]
Operating revenue									375,340	380,210	375,851
Operating income:
Operating income									35,067	26,326	27,438
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue									322,324	336,596	339,967
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue									53,016	43,614	35,884
Dedicated [Member]
Operating revenue									223,852	166,881	157,370
Operating income:
Operating income									18,589	17,074	19,550
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue									187,137	153,691	147,007
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue									36,715	13,190	10,363
Intermodal [Member]
Operating revenue									102,025	80,621	71,490
Operating income:
Operating income									11,150	8,303	7,131
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue									85,572	70,282	64,508
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue									16,453	10,339	6,982
Brokerage [Member]
Operating revenue									86,377	70,408	66,433
Operating income:
Operating income									$ 5,542	$ 5,159	$ 4,184