ACQUISITIONS (Details)						3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 06, 2018 USD ($) $ / shares shares	Dec. 01, 2017 USD ($) shares	Sep. 01, 2017 USD ($)	Jul. 01, 2017 USD ($) shares	May 01, 2017 USD ($) shares	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($) item $ / shares	Jun. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 29, 2017 USD ($)
ACQUISITIONS
Equity transferred						$ 15,355,000	$ 3,438,000	$ 15,355,000	$ 3,438,000
Revenue						376,899,000	197,323,000	704,480,000	357,757,000
Net income						13,485,000	(4,107,000)	12,688,000	(11,852,000)
Allocation of the purchase price paid to the fair values of the net assets
Goodwill						264,025,000		264,025,000		$ 302,702,000
Amortization of intangible assets						6,239,000	1,442,000	8,133,000	2,884,000
Accumulated amortization						37,805,000		37,805,000		29,686,000
Provisional income tax benefit due to TCJA						14,000,000		14,000,000
Non-competition agreements
Allocation of the purchase price paid to the fair values of the net assets
Accumulated amortization						9,316,000		9,316,000		5,765,000
Customer relationships
Allocation of the purchase price paid to the fair values of the net assets
Accumulated amortization						28,489,000		$ 28,489,000		$ 23,921,000
Minimum
ACQUISITIONS
Number of open-deck trucking companies acquired | item								17
Belmont
ACQUISITIONS
Voting interest acquired (as a percent)										100.00%
Total consideration paid										$ 4,600,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable												$ 240,000
Prepaid and other current assets												107,000
Property and equipment												1,548,000
Goodwill & Intangibles												4,187,000
Intangible assets						1,700,000		$ 1,700,000
Deferred tax liabilities												(1,300,000)
Accounts payable and other liabilities												(248,000)
Total												$ 4,534,000
Amortization of intangible assets						100,000
Accumulated amortization						100,000		100,000			$ 62,566
Increase (decrease) to goodwill due to valuation								(1,700,000)
Increase (decrease) in goodwill								600,000
Decrease in deferred tax liability								600,000
Provisional income tax benefit due to TCJA								400,000
Belmont | Non-competition agreements
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						200,000		200,000
Belmont | Customer relationships
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						1,200,000		1,200,000
Belmont | Trade names
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						300,000		300,000
Moore Freight Service
ACQUISITIONS
Voting interest acquired (as a percent)		100.00%
Total consideration paid		$ 59,100,000
Cash payments for business consideration		$ 35,100,000
Number of shares transferred | shares		145,129
Equity transferred		$ 1,800,000
Outstanding debt assumed		22,200,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable		4,436,000
Parts supplies		312,000
Prepaid and other current assets		301,000
Property and equipment		22,052,000
Goodwill & Intangibles		48,220,000
Other long-term assets		114,000
Intangible assets						30,400,000		30,400,000
Deferred tax liabilities		(14,376,000)
Accounts payable and other liabilities		(1,889,000)
Total		59,170,000
Transaction expenses incurred, not deductible for taxes purposes		$ 600,000
Amortization of intangible assets						1,500,000
Accumulated amortization											900,000
Increase (decrease) to goodwill due to valuation								(30,400,000)
Increase (decrease) in goodwill								11,700,000
Decrease in deferred tax liability								11,700,000
Provisional income tax benefit due to TCJA								4,000,000
Moore Freight Service | Non-competition agreements
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						3,500,000		3,500,000
Moore Freight Service | Customer relationships
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						23,700,000		23,700,000
Moore Freight Service | Trade names
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						3,200,000		3,200,000
Roadmaster Group
ACQUISITIONS
Voting interest acquired (as a percent)		100.00%
Total consideration paid		$ 86,900,000
Cash payments for business consideration		$ 37,500,000
Number of shares transferred | shares		3,114,247
Equity transferred		$ 39,100,000
Outstanding debt assumed		10,300,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable		9,806,000
Parts supplies		231,000
Prepaid and other current assets		1,097,000
Property and equipment		36,854,000
Goodwill & Intangibles		83,947,000
Other long-term assets		670,000
Intangible assets						22,900,000		22,900,000
Deferred tax liabilities		(19,410,000)
Accounts payable and other liabilities		(26,764,000)
Total		86,431,000
Transaction expenses incurred, not deductible for taxes purposes		$ 600,000
Amortization of intangible assets						600,000
Accumulated amortization						600,000		600,000			300,000
Increase (decrease) to goodwill due to valuation								(22,900,000)
Increase (decrease) in goodwill								8,700,000
Decrease in deferred tax liability								8,700,000
Provisional income tax benefit due to TCJA								3,000,000
Roadmaster Group | Non-competition agreements
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						2,900,000		2,900,000
Roadmaster Group | Customer relationships
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						7,300,000		7,300,000
Roadmaster Group | Trade names
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						12,700,000		12,700,000
Tennessee Steel Haulers and Co
ACQUISITIONS
Voting interest acquired (as a percent)		100.00%
Total consideration paid		$ 91,900,000
Cash payments for business consideration		$ 74,900,000
Number of shares transferred | shares		972,680
Equity transferred		$ 12,000,000
Outstanding debt assumed		5,000,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable		20,207,000
Prepaid and other current assets		5,870,000
Property and equipment		8,705,000
Goodwill & Intangibles		85,102,000
Other long-term assets		19,049,000
Intangible assets						49,800,000		49,800,000
Deferred tax liabilities		(33,284,000)
Accounts payable and other liabilities		(14,002,000)
Total		91,647,000
Transaction expenses incurred, not deductible for taxes purposes		$ 500,000
Amortization of intangible assets						2,100,000
Accumulated amortization											$ 1,200,000
Increase (decrease) to goodwill due to valuation								(49,800,000)
Increase (decrease) in goodwill								19,200,000
Decrease in deferred tax liability								19,200,000
Provisional income tax benefit due to TCJA								6,600,000
Tennessee Steel Haulers and Co | Non-competition agreements
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						12,400,000		12,400,000
Tennessee Steel Haulers and Co | Customer relationships
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						15,900,000		15,900,000
Tennessee Steel Haulers and Co | Trade names
Allocation of the purchase price paid to the fair values of the net assets
Intangible assets						21,500,000		21,500,000
RandR Trucking Holdings, LLC
ACQUISITIONS
Voting interest acquired (as a percent)			100.00%
Total consideration paid			$ 38,400,000
Cash payments for business consideration			24,600,000
Outstanding debt assumed			13,800,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable			5,129,000
Parts supplies			149,000
Prepaid and other current assets			1,515,000
Property and equipment			16,887,000
Goodwill & Intangibles			26,686,000
Other long-term assets			156,000
Deferred tax liabilities			(8,922,000)
Accounts payable and other liabilities			(3,362,000)
Total			38,238,000
Transaction expenses incurred, not deductible for taxes purposes			$ 600,000
The Steelman Companies
ACQUISITIONS
Voting interest acquired (as a percent)				100.00%
Total consideration paid				$ 18,800,000
Cash payments for business consideration				$ 11,200,000
Number of shares transferred | shares				746,170
Recognized Identifiable Assets Acquired And Liabilities Assumed Intangible Assets				$ 14,900,000
Equity transferred				7,600,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable				4,383,000
Parts supplies				90,000
Prepaid and other current assets				2,295,000
Property and equipment				11,100,000
Goodwill & Intangibles				16,367,000
Other long-term assets				5,013,000
Deferred tax liabilities				(4,811,000)
Accounts payable and other liabilities				(15,612,000)
Total				18,825,000
Transaction expenses incurred, not deductible for taxes purposes				$ 300,000
Increase (decrease) in goodwill								1,700,000
Decrease in deferred tax liability								1,700,000
Schilli Transportation Services Inc
ACQUISITIONS
Voting interest acquired (as a percent)					100.00%
Total consideration paid					$ 27,400,000
Cash payments for business consideration					$ 21,000,000
Number of shares transferred | shares					232,885
Equity transferred					$ 2,300,000
Outstanding debt assumed					4,000,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable					8,616,000
Parts supplies					1,681,000
Prepaid and other current assets					2,548,000
Property and equipment					39,870,000
Goodwill & Intangibles					17,050,000
Other long-term assets					915,000
Deferred tax liabilities					(15,424,000)
Accounts payable and other liabilities					(27,896,000)
Total					27,360,000
Transaction expenses incurred, not deductible for taxes purposes					$ 400,000
Increase (decrease) to goodwill due to valuation								2,800,000
Increase (decrease) in goodwill								(700,000)
Decrease in receivables								900,000
Decrease in assets held-for-sale								300,000
Decrease in fixed assets								1,600,000
Decrease in deferred tax liability								(700,000)
Big Freight Systems Inc
ACQUISITIONS
Voting interest acquired (as a percent)					100.00%
Total consideration paid					$ 16,700,000
Cash payments for business consideration					$ 12,400,000
Number of shares transferred | shares					109,248
Equity transferred					$ 1,100,000
EBITDA multiplier					400,000
Contingent liability					1,100,000
Outstanding debt assumed					3,200,000
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable					4,914,000
Parts supplies					212,000
Prepaid and other current assets					287,000
Property and equipment					11,493,000
Goodwill & Intangibles					11,848,000
Other long-term assets					121,000
Deferred tax liabilities					(4,792,000)
Accounts payable and other liabilities					(6,294,000)
Total					17,789,000
Transaction expenses incurred, not deductible for taxes purposes					$ 600,000
Increase (decrease) in goodwill								600,000
Foreign currency translation adjustment								300,000
Decrease in deferred tax liability								600,000
Aveda
ACQUISITIONS
Total consideration paid	$ 117,800,000
Cash payments for business consideration	$ 27,300,000
Number of shares transferred | shares	1,612,979
Equity transferred	$ 15,400,000
Contingent liability	20,300,000
Outstanding debt assumed	$ 54,800,000
Revenue						15,000,000		15,000,000
Net income						(700,000)		(700,000)
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable						37,255,000		37,255,000
Prepaid and other current assets						1,785,000		1,785,000
Property and equipment						89,752,000		89,752,000
Goodwill						14,919,000		14,919,000
Deferred tax asset						1,035,000		1,035,000
Accounts payable and other liabilities						(30,007,000)		(30,007,000)
Total						114,739,000		114,739,000
Cash per common share | $ / shares	$ 0.45
Transaction expenses incurred, not deductible for taxes purposes	$ 1,100,000
Business Acquisition, Pro Forma Information [Abstract]
Pro forma revenue						413,200,000	236,100,000	773,300,000	427,400,000
Pro forma net loss						$ 9,600,000	$ (5,100,000)	$ 8,600,000	$ (15,400,000)
Pro forma Basic | $ / shares						$ 0.16	$ (0.13)	$ 0.15	$ (0.47)
Pro forma Diluted | $ / shares						$ (0.05)	$ (0.13)	$ (0.08)	$ (0.47)