UNAUDITED 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]
Revenue	$ 921,163	$ 894,734	$ 1,854,161	$ 1,732,076
Transportation costs	788,460	793,743	1,594,169	1,540,046
Gross margin	132,703	100,991	259,992	192,030
Costs and expenses:
Salaries and benefits	60,859	54,068	122,887	106,373
General and administrative	24,028	17,794	46,946	36,230
Depreciation and amortization	7,095	3,723	13,849	7,486
Total costs and expenses	91,982	75,585	183,682	150,089
Operating income	40,721	25,406	76,310	41,941
Other income (expense):
Interest expense	(2,690)	(2,187)	(5,746)	(4,291)
Interest income	595	14	968	25
Other, net	(30)	(182)	(70)	(231)
Total other expense	(2,125)	(2,355)	(4,848)	(4,497)
Income from continuing operations before income taxes	38,596	23,051	71,462	37,444
Income tax expense	9,379	5,897	18,351	9,221
Income from continuing operations	29,217	17,154	53,111	28,223
Income from discontinued operations, net of income taxes		4,897		9,995
Net income	29,217	22,051	53,111	38,218
Other comprehensive income:
Foreign currency translation adjustments	(3)	(25)	4	4
Total comprehensive income	$ 29,214	$ 22,026	$ 53,115	$ 38,222
Earnings per share from continuing operations
Basic	$ 0.87	$ 0.51	$ 1.58	$ 0.84
Diluted	0.87	0.51	1.58	0.84
Earnings per share from discontinued operations
Basic		0.15		0.30
Diluted		0.15		0.30
Earnings per share net income
Basic	0.87	0.66	1.58	1.14
Diluted	$ 0.87	$ 0.66	$ 1.58	$ 1.14
Basic weighted average number of shares outstanding	33,552	33,389	33,560	33,382
Diluted weighted average number of shares outstanding	33,715	33,562	33,650	33,520