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		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jan. 01, 2018
Operating revenue		$ 197,024	$ 171,511	$ 383,984		$ 344,670
Operating expenses:
Salaries, wages and benefits		63,250	56,715	122,072		113,115
Purchased transportation		35,048	27,516	70,074		56,878
Fuel and fuel taxes		31,742	25,007	60,786		50,963
Supplies and maintenance		10,251	10,541	20,687		21,531
Income taxes expense		4,659	6,303	8,111		11,886
Net income		13,702	9,141	24,033	$ 72,929	17,355
Operating revenue		197,024	$ 171,511	383,984		$ 344,670
Assets:
Prepaid expenses and other		19,876		19,876	19,810		$ 22,255
Liabilities:
Accounts payable and accrued liabilities		45,782		45,782	38,100		40,060
Stockholders’ equity:
Retained earnings		470,329		470,329	$ 448,542		$ 449,027
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Operating revenue		197,113		384,168
Operating expenses:
Salaries, wages and benefits		63,262		122,020
Purchased transportation		35,106		69,929
Fuel and fuel taxes		31,740		60,783
Supplies and maintenance		10,300		20,711
Income taxes expense		4,652		8,211
Net income		13,681		24,293
Operating revenue		197,113		384,168
Assets:
Prepaid expenses and other		17,615		17,615
Liabilities:
Accounts payable and accrued liabilities		43,746		43,746
Stockholders’ equity:
Retained earnings		470,104		470,104
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating revenue		(89)		(184)
Operating expenses:
Salaries, wages and benefits		(12)		52
Purchased transportation		(58)		145
Fuel and fuel taxes		2		3
Supplies and maintenance		(49)		(24)
Income taxes expense		7		(100)
Net income		21		(260)
Operating revenue		(89)		(184)
Assets:
Prepaid expenses and other	[1]	2,261		2,261
Liabilities:
Accounts payable and accrued liabilities		2,036		2,036
Stockholders’ equity:
Retained earnings		$ 225		$ 225

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