CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Revenue	$ 4,232,383	$ 3,495,644	$ 3,668,117
Transportation costs	3,632,743	3,070,207	3,147,047
Gross margin	599,640	425,437	521,070
Costs and expenses:
Salaries and benefits	247,240	188,777	235,963
General and administrative	76,476	99,597	104,206
Depreciation and amortization	37,467	31,237	28,481
Total costs and expenses	361,183	319,611	368,650
Operating income	238,457	105,826	152,420
Other income (expense):
Interest expense	(7,307)	(9,746)	(10,994)
Interest income	5	403	2,103
Other, net	(245)	(383)	341
Total other income (expense)	(7,547)	(9,726)	(8,550)
Income from continuing operations before income taxes	230,910	96,100	143,870
Income tax expense	59,436	22,541	36,699
Net income	171,474	73,559	107,171
Other comprehensive (loss) income:
Foreign currency translation adjustments	(16)	(5)	(4)
Total comprehensive income	$ 171,458	$ 73,554	$ 107,167
Earnings per share from continuing operations
Basic	$ 5.13	$ 2.22	$ 3.22
Diluted	5.06	2.19	3.20
Earnings per share net income
Basic	5.13	2.22	3.22
Diluted	$ 5.06	$ 2.19	$ 3.20
Basic weighted average number of shares outstanding	33,434	33,180	33,284
Diluted weighted average number of shares outstanding	33,892	33,543	33,480