CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue	$ 4,034,897	$ 3,572,790	$ 3,525,595
Transportation costs	3,577,380	3,118,005	3,112,900
Gross margin	457,517	454,785	412,695
Costs and expenses:
Salaries and benefits	188,389	180,459	158,938
Agent fees and commissions	74,082	72,896	68,724
General and administrative	85,182	68,630	60,015
Depreciation and amortization	13,313	8,966	7,988
Total costs and expenses	360,966	330,951	295,665
Operating income	96,551	123,834	117,030
Other income (expense):
Interest expense	(6,754)	(3,625)	(2,971)
Interest and dividend income	416	393	83
Other, net	724	819	(2,560)
Total other expense	(5,614)	(2,413)	(5,448)
Income before provision for income taxes	90,937	121,421	111,582
Income tax (benefit) expense	(44,216)	46,616	40,633
Net income	135,153	74,805	70,949
Other comprehensive income (loss):
Foreign currency translation adjustments	79	(95)	(101)
Total comprehensive income	$ 135,232	$ 74,710	$ 70,848
Basic earnings per common share	$ 4.07	$ 2.21	$ 1.98
Diluted earnings per common share	$ 4.05	$ 2.20	$ 1.97
Basic weighted average number of shares outstanding	33,220	33,841	35,876
Diluted weighted average number of shares outstanding	33,350	33,949	35,968