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Summary of Operating Results (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Segment Reporting Information [Line Items]        
Revenue $ 836,685 $ 778,312 $ 1,605,665 $ 1,518,197
Transportation costs 741,212 690,954 1,422,854 1,347,097
Gross margin 95,473 87,358 182,811 171,100
Costs and expenses:        
Salaries and benefits 35,002 31,436 69,585 64,735
Agent fees and commissions 13,686 13,601 26,960 27,296
General and administrative 14,728 12,734 27,919 25,311
Depreciation and amortization 1,593 1,737 3,146 3,397
Total costs and expenses 65,009 59,508 127,610 120,739
Operating income 30,464 27,850 55,201 50,361
Capital expenditures 29,746 4,030 39,202 21,043
Operating Segments [Member] | Hub [Member]
        
Segment Reporting Information [Line Items]        
Revenue 644,924 595,888 1,237,575 1,159,100
Transportation costs 573,498 531,080 1,100,969 1,032,773
Gross margin 71,426 64,808 136,606 126,327
Costs and expenses:        
Salaries and benefits 31,208 27,590 61,985 56,671
Agent fees and commissions 350 490 799 1,125
General and administrative 13,338 10,707 25,036 21,438
Depreciation and amortization 1,061 1,099 2,081 2,221
Total costs and expenses 45,957 39,886 89,901 81,455
Operating income 25,469 24,922 46,705 44,872
Capital expenditures 29,626 3,580 38,155 20,322
Operating Segments [Member] | Mode [Member]
        
Segment Reporting Information [Line Items]        
Revenue 203,918 194,292 391,378 381,475
Transportation costs 179,871 171,742 345,173 336,702
Gross margin 24,047 22,550 46,205 44,773
Costs and expenses:        
Salaries and benefits 3,794 3,846 7,600 8,064
Agent fees and commissions 13,336 13,111 26,161 26,171
General and administrative 1,390 2,027 2,883 3,873
Depreciation and amortization 532 638 1,065 1,176
Total costs and expenses 19,052 19,622 37,709 39,284
Operating income 4,995 2,928 8,496 5,489
Capital expenditures 120 450 1,047 721
Intersegment Elims [Member]
        
Segment Reporting Information [Line Items]        
Revenue (12,157) (11,868) (23,288) (22,378)
Transportation costs $ (12,157) $ (11,868) $ (23,288) $ (22,378)