UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenue	$ 1,179,093	$ 923,596	$ 2,275,064	$ 1,816,352
Transportation costs	1,042,903	822,279	2,016,083	1,613,450
Gross margin	136,190	101,317	258,981	202,902
Costs and expenses:
Salaries and benefits	58,092	45,038	114,099	88,217
Agent fees and commissions	21,969	17,038	41,111	35,031
General and administrative	20,216	20,114	40,720	40,938
Depreciation and amortization	3,960	2,549	7,965	4,961
Total costs and expenses	104,237	84,739	203,895	169,147
Operating income	31,953	16,578	55,086	33,755
Other income (expense):
Interest expense	(2,187)	(1,032)	(4,291)	(2,130)
Interest and dividend income	22	200	44	330
Other, net	(189)		(244)	194
Total other expense	(2,354)	(832)	(4,491)	(1,606)
Income before provision for income taxes	29,599	15,746	50,595	32,149
Provision for income taxes	7,548	6,204	12,377	12,273
Net income	22,051	9,542	38,218	19,876
Other comprehensive income:
Foreign currency translation adjustments	(25)	28	4	92
Total comprehensive income	$ 22,026	$ 9,570	$ 38,222	$ 19,968
Basic earnings per common share	$ 0.66	$ 0.29	$ 1.14	$ 0.60
Diluted earnings per common share	$ 0.66	$ 0.29	$ 1.14	$ 0.60
Basic weighted average number of shares outstanding	33,389	33,220	33,382	33,213
Diluted weighted average number of shares outstanding	33,562	33,262	33,520	33,318