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
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jan. 01, 2018
Operating revenue		$ 186,960	$ 173,159
Salaries, wages and benefits		58,822	56,400
Purchased transportation		35,026	29,362
Fuel and fuel taxes		29,044	25,956
Supplies and maintenance		10,436	10,990
Income taxes expense		3,452	5,583
Net income		10,331	$ 8,214	$ 82,070
Assets:
Prepaid expenses and other		19,186		19,810	$ 22,255
Liabilities:
Accounts payable and accrued liabilities		49,421		38,100	40,060
Stockholders’ equity:
Retained earnings		457,993		448,542	$ 449,027
Accounting Standards Update 2014-09 [Member]
Assets:
Prepaid expenses and other	[1]			2,445
Liabilities:
Accounts payable and accrued liabilities				1,960
Stockholders’ equity:
Retained earnings				$ 485
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Operating revenue		187,055
Salaries, wages and benefits		58,758
Purchased transportation		34,823
Fuel and fuel taxes		29,043
Supplies and maintenance		10,411
Income taxes expense		3,559
Net income		10,612
Assets:
Prepaid expenses and other		16,836
Liabilities:
Accounts payable and accrued liabilities		47,275
Stockholders’ equity:
Retained earnings		457,789
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating revenue		(95)
Salaries, wages and benefits		64
Purchased transportation		203
Fuel and fuel taxes		1
Supplies and maintenance		25
Income taxes expense		(107)
Net income		(281)
Assets:
Prepaid expenses and other	[2]	2,350
Liabilities:
Accounts payable and accrued liabilities		2,146
Stockholders’ equity:
Retained earnings		$ 204

[1] Contract assets balance at January 1, 2018.

[2] Contract assets balance at March 31, 2018.