Note 15 - Business Segments (Details) - Operating Revenue and Operating Income by Segment (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating Revenue	$ 166,224	$ 167,103	$ 161,413	$ 164,474	$ 166,412	$ 163,606	$ 156,964	$ 151,474	$ 659,214		$ 638,456		$ 603,679
Operating Income	12,683	13,653	13,121	12,538	13,211	9,840	13,283	9,519	51,995		45,853		43,030
Truckload Revenue, Net of Fuel Surcharge [Member]
Segment Reporting Information [Line Items]
Operating Revenue									400,324		378,383		357,787
Truckload Fuel Surcharge Revenue [Member]
Segment Reporting Information [Line Items]
Operating Revenue									106,955		104,940		98,060
Total Truckload Revenue [Member]
Segment Reporting Information [Line Items]
Operating Revenue									507,279		483,323		455,847
Logistics Revenue, Net of Intermodal Fuel Surcharge [Member]
Segment Reporting Information [Line Items]
Operating Revenue									131,186	[1]	138,960	[1]	132,890	[1]
Intermodal Fuel Surcharge Revenue [Member]
Segment Reporting Information [Line Items]
Operating Revenue									20,749		16,173		14,942
Total Logistics Revenue [Member]
Segment Reporting Information [Line Items]
Operating Revenue									151,935		155,133		147,832
Truckload [Member]
Segment Reporting Information [Line Items]
Operating Income									44,865		36,805		35,298
Logistics [Member]
Segment Reporting Information [Line Items]
Operating Income									$ 7,130		$ 9,048		$ 7,732

[1] Logistics revenue is net of $2.1 million, $9.7 million and $9.0 million of inter-segment revenue in 2013, 2012 and 2011, respectively, for loads transported by our tractors and arranged by MWL that have been eliminated in consolidation. The inter-segment revenue in 2013 relates to loads transported prior to the deconsolidation of MWL effective March 28, 2013.