CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenue	$ 3,572,790	$ 3,525,595	$ 3,571,126
Transportation costs	3,118,005	3,112,900	3,200,691
Gross margin	454,785	412,695	370,435
Costs and expenses:
Salaries and benefits	180,459	158,938	136,480
Agent fees and commissions	72,896	68,724	62,041
General and administrative	68,630	60,015	58,014
Depreciation and amortization	8,966	7,988	7,799
Driver settlements and related costs			10,343
Impairment of software			11,881
Total costs and expenses	330,951	295,665	286,558
Operating income	123,834	117,030	83,877
Other income (expense):
Interest expense	(3,625)	(2,971)	(1,785)
Interest and dividend income	393	83	32
Other, net	819	(2,560)	(257)
Total other expense	(2,413)	(5,448)	(2,010)
Income before provision for income taxes	121,421	111,582	81,867
Provision for income taxes	46,616	40,633	30,309
Net income	74,805	70,949	51,558
Other comprehensive (loss) income:
Foreign currency translation adjustments	(95)	(101)	8
Total comprehensive income	$ 74,710	$ 70,848	$ 51,566
Basic earnings per common share	$ 2.21	$ 1.98	$ 1.41
Diluted earnings per common share	$ 2.20	$ 1.97	$ 1.40
Basic weighted average number of shares outstanding	33,841	35,876	36,590
Diluted weighted average number of shares outstanding	33,949	35,968	36,732