Note 2 - Adoption of New Accounting Standard - Impact of New Standard on Consolidated Condensed Balance Sheet and Statements of Operations (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018
Operating revenue		$ 199,649		$ 170,679	$ 583,633	$ 515,349
Salaries, wages and benefits		64,051		53,594	186,123	166,709
Purchased transportation		35,867		28,668	105,941	85,546
Fuel and fuel taxes		31,658		26,143	92,444	77,106
Supplies and maintenance		10,574		10,381	31,261	31,912
Income taxes expense		3,856		5,153	11,967	17,039
Net income		15,257	$ 65,074	$ 7,855	39,290	$ 25,210
Prepaid expenses and other		18,816	19,810		18,816		$ 22,255
Accounts payable and accrued liabilities		48,259	38,100		48,259		40,060
Retained earnings		484,219	$ 448,542		484,219		$ 449,027
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Operating revenue		199,100			583,268
Salaries, wages and benefits		63,935			185,955
Purchased transportation		35,568			105,497
Fuel and fuel taxes		31,627			92,410
Supplies and maintenance		10,551			31,262
Income taxes expense		3,835			12,046
Net income		15,198			39,491
Prepaid expenses and other		16,006			16,006
Accounts payable and accrued liabilities		45,733			45,733
Retained earnings		483,935			483,935
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating revenue		549			365
Salaries, wages and benefits		116			168
Purchased transportation		299			444
Fuel and fuel taxes		31			34
Supplies and maintenance		23			(1)
Income taxes expense		21			(79)
Net income		59			(201)
Prepaid expenses and other	[1]	2,810			2,810
Accounts payable and accrued liabilities		2,526			2,526
Retained earnings		$ 284			$ 284

[1] Contract assets balance at September 30, 2018.