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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Operating revenue:
Operating Revenue	$ 173,547	$ 171,550	$ 168,423	$ 159,409	$ 166,224	$ 167,103	$ 161,413	$ 164,474	$ 672,929		$ 659,214		$ 638,456
Operating income:
Operating Income	16,392	13,042	12,932	8,640	12,683	13,653	13,121	12,538	51,006		51,995		45,853
Revenue, Net of Fuel Surchage [Member] | Truckload [Member]
Operating revenue:
Operating Revenue									358,458		366,798		363,645
Revenue, Net of Fuel Surchage [Member] | Dedicated [Member]
Operating revenue:
Operating Revenue									56,609		33,526		14,738
Revenue, Net of Fuel Surchage [Member] | Intermodal [Member]
Operating revenue:
Operating Revenue									75,447		71,764		55,151
Fuel Surcharge Revenue [Member] | Truckload [Member]
Operating revenue:
Operating Revenue									89,815		98,161		100,067
Fuel Surcharge Revenue [Member] | Dedicated [Member]
Operating revenue:
Operating Revenue									13,743		8,794		4,873
Fuel Surcharge Revenue [Member] | Intermodal [Member]
Operating revenue:
Operating Revenue									21,645		20,749		16,173
Truckload [Member]
Operating revenue:
Operating Revenue									448,273		464,959		463,712
Operating income:
Operating Income									39,483		39,290		33,715
Dedicated [Member]
Operating revenue:
Operating Revenue									70,352		42,320		19,611
Operating income:
Operating Income									7,136		5,575		3,090
Intermodal [Member]
Operating revenue:
Operating Revenue									97,092		92,513		71,324
Operating income:
Operating Income									1,735		4,014		4,703
Brokerage [Member] | Marten Transport [Member]
Operating revenue:
Operating Revenue									57,212		52,746		53,161
Brokerage [Member] | MWL [Member]
Operating revenue:
Operating Revenue										[1]	6,676	[1]	30,648	[1]
Brokerage [Member]
Operating revenue:
Operating Revenue									57,212		59,422		83,809
Operating income:
Operating Income									$ 2,652		$ 3,116		$ 4,345

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