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								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Operating revenue										$ 787,594	$ 698,120	$ 671,144
Operating expenses (income):
Salaries, wages and benefits										252,047	226,091	224,827
Purchased transportation										144,611	118,349	110,719
Fuel and fuel taxes										121,633	105,390	94,075
Supplies and maintenance										40,853	41,613	43,912
Income Tax Expense (Benefit), Total										16,002	(33,811)	23,602
Net income		$ 15,737	$ 15,257	$ 13,702	$ 10,331	$ 65,074	$ 7,855	$ 9,141	$ 8,214	55,027	90,284	$ 33,464
Assets:
Prepaid expenses and other		19,924				19,810				19,924	19,810		$ 22,255
Contract assets		1,823								1,823
Liabilities:
Accounts payable and accrued liabilities		43,870				38,100				43,870	38,100		40,060
Stockholders’ equity:
Retained earnings		498,595				$ 448,542				498,595	$ 448,542		$ 449,027
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Operating revenue										788,216
Operating expenses (income):
Salaries, wages and benefits										252,327
Purchased transportation										144,573
Fuel and fuel taxes										121,666
Supplies and maintenance										40,832
Income Tax Expense (Benefit), Total										16,121
Net income										55,276
Assets:
Prepaid expenses and other		18,101								18,101
Liabilities:
Accounts payable and accrued liabilities		42,283								42,283
Stockholders’ equity:
Retained earnings		498,359								498,359
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating revenue										(622)
Operating expenses (income):
Salaries, wages and benefits										(280)
Purchased transportation										38
Fuel and fuel taxes										(33)
Supplies and maintenance										21
Income Tax Expense (Benefit), Total										(119)
Net income										(249)
Assets:
Contract assets	[1]	1,823								1,823
Liabilities:
Accounts payable and accrued liabilities		1,587								1,587
Stockholders’ equity:
Retained earnings		$ 236								$ 236

[1] Contract assets balance at December 31, 2018.