UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenue	$ 779,243	$ 921,163	$ 1,618,102	$ 1,854,161
Transportation costs	671,994	788,460	1,406,259	1,594,169
Gross margin	107,249	132,703	211,843	259,992
Costs and expenses:
Salaries and benefits	49,676	60,859	100,552	122,887
General and administrative	28,970	24,028	55,306	46,946
Depreciation and amortization	7,625	7,095	15,248	13,849
Total costs and expenses	86,271	91,982	171,106	183,682
Operating income	20,978	40,721	40,737	76,310
Other income (expense):
Interest expense	(3,006)	(2,690)	(5,461)	(5,746)
Other, net	47	565	228	898
Total other expense	(2,959)	(2,125)	(5,233)	(4,848)
Income before provision for income taxes	18,019	38,596	35,504	71,462
Income tax expense	4,865	9,379	9,114	18,351
Net income	13,154	29,217	26,390	53,111
Other comprehensive income:
Foreign currency translation adjustments	37	(3)	(97)	4
Total comprehensive income	$ 13,191	$ 29,214	$ 26,293	$ 53,115
Basic earnings per common share	$ 0.40	$ 0.87	$ 0.80	$ 1.58
Diluted earnings per common share	$ 0.39	$ 0.87	$ 0.79	$ 1.58
Basic weighted average number of shares outstanding	33,171	33,552	33,165	33,560
Diluted weighted average number of shares outstanding	33,455	33,715	33,472	33,650