Summary of Operating Results (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenue	$ 1,162,576	$ 1,054,360	$ 924,513	$ 893,448	$ 978,560	$ 932,814	$ 855,557	$ 805,859	$ 4,034,897	$ 3,572,790	$ 3,525,595
Transportation costs									3,577,380	3,118,005	3,112,900
Gross margin	138,091	116,524	101,317	101,585	120,454	111,454	114,490	108,387	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	$ 41,127	$ 21,669	$ 16,578	$ 17,177	$ 30,839	$ 29,855	$ 34,297	$ 28,843	96,551	123,834	117,030
Capital expenditures									74,541	107,409	83,042
Operating Segments [Member] | Hub [Member]
Segment Reporting Information [Line Items]
Revenue									3,107,394	2,734,541	2,679,318
Transportation costs									2,771,291	2,404,946	2,385,197
Gross margin									336,103	329,595	294,121
Costs and expenses:
Salaries and benefits									174,573	165,136	143,993
Agent fees and commissions									58	66	56
General and administrative									77,085	60,811	53,023
Depreciation and amortization									12,139	7,698	6,688
Total costs and expenses									263,855	233,711	203,760
Operating income									72,248	95,884	90,361
Capital expenditures									73,772	106,316	79,860
Operating Segments [Member] | Mode LLC [Member]
Segment Reporting Information [Line Items]
Revenue									1,029,160	948,735	928,661
Transportation costs									907,746	823,545	810,087
Gross margin									121,414	125,190	118,574
Costs and expenses:
Salaries and benefits									13,816	15,323	14,945
Agent fees and commissions									74,024	72,830	68,668
General and administrative									8,097	7,819	6,992
Depreciation and amortization									1,174	1,268	1,300
Total costs and expenses									97,111	97,240	91,905
Operating income									24,303	27,950	26,669
Capital expenditures									769	1,093	3,182
Intersegment Elims [Member]
Segment Reporting Information [Line Items]
Revenue									(101,657)	(110,486)	(82,384)
Transportation costs									$ (101,657)	$ (110,486)	$ (82,384)