REVENUE RECOGNITION - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue
Total revenue	$ 376,899	$ 197,323	$ 704,480	$ 357,757
Corporate/Eliminations
Disaggregation of Revenue
Total revenue	(3,638)	(1,560)	(5,953)	(3,103)
Freight
Disaggregation of Revenue
Total revenue	272,583	149,654	512,654	275,209
Freight | Corporate/Eliminations
Disaggregation of Revenue
Total revenue	(3,021)	(1,378)	(4,971)	(2,772)
Brokerage
Disaggregation of Revenue
Total revenue	60,091	28,656	106,229	49,525
Brokerage | Corporate/Eliminations
Disaggregation of Revenue
Total revenue	(183)	(37)	(233)	(36)
Logistics
Disaggregation of Revenue
Total revenue	8,891	2,700	19,609	2,700
Logistics | Corporate/Eliminations
Disaggregation of Revenue
Total revenue	(38)	(8)	(71)	(8)
Fuel
Disaggregation of Revenue
Total revenue	35,334	16,313	65,988	30,323
Fuel | Corporate/Eliminations
Disaggregation of Revenue
Total revenue	(396)	(137)	(678)	(287)
Flatbed | Operating Segments
Disaggregation of Revenue
Total revenue	162,179	86,898	307,187	168,202
Flatbed | Freight | Operating Segments
Disaggregation of Revenue
Total revenue	118,486	68,889	222,998	132,863
Flatbed | Brokerage | Operating Segments
Disaggregation of Revenue
Total revenue	23,875	9,495	46,873	18,593
Flatbed | Logistics | Operating Segments
Disaggregation of Revenue
Total revenue	693		1,407
Flatbed | Fuel | Operating Segments
Disaggregation of Revenue
Total revenue	19,125	8,514	35,909	16,746
Specialized | Operating Segments
Disaggregation of Revenue
Total revenue	218,358	111,985	403,246	192,658
Specialized | Freight | Operating Segments
Disaggregation of Revenue
Total revenue	157,118	82,143	294,627	145,118
Specialized | Brokerage | Operating Segments
Disaggregation of Revenue
Total revenue	36,399	19,198	59,589	30,968
Specialized | Logistics | Operating Segments
Disaggregation of Revenue
Total revenue	8,236	2,708	18,273	2,708
Specialized | Fuel | Operating Segments
Disaggregation of Revenue
Total revenue	$ 16,605	$ 7,936	$ 30,757	$ 13,864