Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Cash flows provided by operating activities:
Net income	$ 18,006	$ 13,718	$ 55,782
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	25,687	25,427
Tires in service amortization	1,642	1,706
Deferred income taxes	1,085	1,768
Share-based payment arrangement compensation expense	336	246
Changes in other current operating items:
Receivables	(4,241)	(782)
Prepaid expenses and other	1,445	930
Accounts payable	677	617
Insurance and claims accruals	2,117	2,695
Accrued and other current liabilities	(1,200)	(1,290)
Net cash provided by operating activities	43,570	43,480
Cash flows used for investing activities:
Other	(37)	(39)
Net cash used for investing activities	(17,417)	(36,632)
Cash flows used for financing activities:
Dividends on common stock	(3,311)	(2,193)
Repurchase and retirement of common stock	0	(597)
Issuance of common stock from share-based payment arrangement exercises	161	1,054
Employee taxes paid in exchange for shares withheld	(547)	(437)
Net cash used for financing activities	(3,697)	(2,173)
Net change in cash and cash equivalents	22,456	4,675
Cash and cash equivalents:
Beginning of period	66,127	31,461	36,136
End of period	88,583	36,136	$ 66,127
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	5,505	11,107
Operating lease assets and liabilities acquired	0	88
Supplemental disclosure of cash flow information:
Income taxes	1,350	0
Interest	0	0
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of property, plant and equipment	(1,984)	(1,555)
Cash flows used for investing activities:
Property, plant and equipment additions	(30,226)	(44,086)
Proceeds from property, plant and equipment dispositions	13,896	9,459
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Property, plant and equipment additions	$ (1,050)	$ (1,966)