UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Revenue	$ 1,095,971	$ 892,756
Transportation costs	973,180	791,171
Gross margin	122,791	101,585
Costs and expenses:
Salaries and benefits	56,007	43,179
Agent fees and commissions	19,142	17,993
General and administrative	20,504	20,824
Depreciation and amortization	4,005	2,412
Total costs and expenses	99,658	84,408
Operating income	23,133	17,177
Other income (expense):
Interest expense	(2,104)	(1,098)
Interest and dividend income	22	130
Other, net	(55)	194
Total other expense	(2,137)	(774)
Income before provision for income taxes	20,996	16,403
Provision for income taxes	4,829	6,069
Net income	16,167	10,334
Other comprehensive income:
Foreign currency translation adjustments	29	64
Total comprehensive income	$ 16,196	$ 10,398
Basic earnings per common share	$ 0.48	$ 0.31
Diluted earnings per common share	$ 0.48	$ 0.31
Basic weighted average number of shares outstanding	33,375	33,205
Diluted weighted average number of shares outstanding	33,478	33,373