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Summary of Operating Results (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Segment Reporting Information [Line Items]    
Revenue $ 768,980 $ 739,885
Transportation costs 681,642 656,143
Gross margin 87,338 83,742
Costs and expenses:    
Salaries and benefits 34,583 33,299
Agent fees and commissions 13,274 13,695
General and administrative 13,191 12,577
Depreciation and amortization 1,553 1,660
Total costs and expenses 62,601 61,231
Operating income 24,737 22,511
Capital expenditures 9,456 17,013
Hub [Member]
    
Segment Reporting Information [Line Items]    
Revenue 592,651 563,212
Transportation costs 527,471 501,694
Gross margin 65,180 61,518
Costs and expenses:    
Salaries and benefits 30,777 29,080
Agent fees and commissions 449 634
General and administrative 11,698 10,732
Depreciation and amortization 1,020 1,121
Total costs and expenses 43,944 41,567
Operating income 21,236 19,951
Capital expenditures 8,529 16,742
Mode [Member]
    
Segment Reporting Information [Line Items]    
Revenue 187,460 187,183
Transportation costs 165,302 164,959
Gross margin 22,158 22,224
Costs and expenses:    
Salaries and benefits 3,806 4,219
Agent fees and commissions 12,825 13,061
General and administrative 1,493 1,845
Depreciation and amortization 533 539
Total costs and expenses 18,657 19,664
Operating income 3,501 2,560
Capital expenditures 927 271
Intersegment Elims [Member]
    
Segment Reporting Information [Line Items]    
Revenue (11,131) (10,510)
Transportation costs $ (11,131) $ (10,510)