CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenue	$ 981,320	$ 779,243	$ 1,900,873	$ 1,618,102
Transportation costs	860,759	671,994	1,671,565	1,406,259
Gross margin	120,561	107,249	229,308	211,843
Costs and expenses:
Salaries and benefits	54,375	49,676	111,326	100,552
General and administrative	20,370	28,970	39,613	55,306
Depreciation and amortization	8,868	7,625	17,370	15,248
Total costs and expenses	83,613	86,271	168,309	171,106
Operating income	36,948	20,978	60,999	40,737
Other income (expense):
Interest expense	(1,859)	(3,006)	(3,764)	(5,461)
Other, net	(192)	47	(284)	228
Total other expense	(2,051)	(2,959)	(4,048)	(5,233)
Income before provision for income taxes	34,897	18,019	56,951	35,504
Income tax expense	8,305	4,865	13,129	9,114
Net income	26,592	13,154	43,822	26,390
Other comprehensive (loss) income:
Foreign currency translation adjustments	25	37	15	(97)
Total comprehensive income	$ 26,617	$ 13,191	$ 43,837	$ 26,293
Basic earnings per common share	$ 0.80	$ 0.40	$ 1.31	$ 0.80
Diluted earnings per common share	$ 0.78	$ 0.39	$ 1.30	$ 0.79
Basic weighted average number of shares outstanding	33,428	33,171	33,423	33,165
Diluted weighted average number of shares outstanding	33,879	33,455	33,827	33,472