Condensed Consolidated Statements of Income and Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Operating revenue	$ 133,622	$ 135,381	$ 267,596	$ 260,394
Operating expenses:
Salaries, wages and employee benefits	33,806	31,645	69,896	63,882
Fuel and fuel taxes	14,102	13,984	27,733	27,463
Depreciation and amortization	9,125	7,477	17,943	14,657
Equipment rent	7,160	5,341	14,440	10,943
Insurance and claims	2,568	2,151	5,288	4,869
Operations and maintenance	8,481	8,913	15,754	16,874
Purchased transportation	49,072	55,817	97,353	104,855
Operating taxes and licenses	1,311	1,262	2,428	1,764
Communications and utilities	719	677	1,486	1,390
Gain on disposal of assets, net	141	(395)	(4)	(564)
Impairments of assets held for sale	367	0	367	0
Reversal of restructuring, impairment and other costs	0	0	0	(639)
Other	4,787	4,198	9,008	8,197
Total operating expenses	131,639	131,070	261,692	253,691
Operating income	1,983	4,311	5,904	6,703
Other expenses:
Interest expense, net	1,595	833	3,336	1,651
Other, net	171	113	308	233
Total other expenses, net	1,766	946	3,644	1,884
Income before income taxes	217	3,365	2,260	4,819
Income tax expense	216	821	758	1,240
Consolidated net income and comprehensive income	$ 1	$ 2,544	$ 1,502	$ 3,579
Net earnings per share:
Average shares outstanding (basic) (in shares)	8,554	8,205	8,479	8,141
Basic earnings per share (in usd per share)	$ 0.00	$ 0.31	$ 0.18	$ 0.44
Average shares outstanding (diluted) (in shares)	8,567	8,227	8,498	8,167
Diluted earnings per share (in usd per shares)	$ 0.00	$ 0.31	$ 0.18	$ 0.44