Note 18 - Revenue and Business Segments - Operating Revenue and Operating Income by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenue	$ 217,185	$ 214,973	$ 212,090	$ 199,023	$ 203,961	$ 199,649	$ 197,024	$ 186,960	$ 843,271	$ 787,594	$ 698,120
Operating income	$ 18,787	$ 20,034	$ 19,944	$ 17,733	$ 19,538	$ 18,993	$ 18,223	$ 13,594	76,498	70,348	56,862
Truckload [Member]
Operating revenue									378,000	375,340	380,210
Operating income									29,666	35,067	26,326
Truckload [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue									329,304	322,324	336,596
Truckload [Member] | Fuel Surcharge Revenue [Member]
Operating revenue									48,696	53,016	43,614
Dedicated [Member]
Operating revenue									265,984	223,852	166,881
Operating income									31,245	18,589	17,074
Dedicated [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue									223,935	187,137	153,691
Dedicated [Member] | Fuel Surcharge Revenue [Member]
Operating revenue									42,049	36,715	13,190
Intermodal [Member]
Operating revenue									90,394	102,025	80,621
Operating income									6,612	11,150	8,303
Intermodal [Member] | Revenue, Net of Fuel Surcharge [Member]
Operating revenue									77,750	85,572	70,282
Intermodal [Member] | Fuel Surcharge Revenue [Member]
Operating revenue									12,644	16,453	10,339
Brokerage [Member]
Operating revenue									108,893	86,377	70,408
Operating income									$ 8,975	$ 5,542	$ 5,159