UNAUDITED CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Revenue	$ 924,513	$ 855,557	$ 1,817,961	$ 1,661,416
Transportation costs	823,196	741,067	1,615,059	1,438,539
Gross margin	101,317	114,490	202,902	222,877
Costs and expenses:
Salaries and benefits	45,038	43,602	88,217	87,465
Agent fees and commissions	17,038	18,360	35,031	35,262
General and administrative	20,114	16,083	40,938	32,727
Depreciation and amortization	2,549	2,148	4,961	4,283
Total costs and expenses	84,739	80,193	169,147	159,737
Operating income	16,578	34,297	33,755	63,140
Other income (expense):
Interest expense	(1,032)	(857)	(2,130)	(1,767)
Interest and dividend income	200	120	330	181
Other, net		216	194	1,152
Total other expense	(832)	(521)	(1,606)	(434)
Income before provision for income taxes	15,746	33,776	32,149	62,706
Provision for income taxes	6,204	13,105	12,273	24,069
Net income	9,542	20,671	19,876	38,637
Other comprehensive income:
Foreign currency translation adjustments	28	(54)	92	(53)
Total comprehensive income	$ 9,570	$ 20,617	$ 19,968	$ 38,584
Basic earnings per common share	$ 0.29	$ 0.61	$ 0.60	$ 1.12
Diluted earnings per common share	$ 0.29	$ 0.61	$ 0.60	$ 1.12
Basic weighted average number of shares outstanding	33,220	33,944	33,213	34,541
Diluted weighted average number of shares outstanding	33,262	34,027	33,318	34,575