ACQUISITIONS (Details) $ in Millions				3 Months Ended				6 Months Ended
	Aug. 01, 2018 USD ($) shares	Jul. 01, 2018 USD ($) shares	Jun. 06, 2018 USD ($) shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) item	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 06, 2018 $ / shares	Jun. 06, 2018 USD ($)
ACQUISITIONS
Common stock issued in acquisitions						$ 15.4			$ 15.4
Revenue				$ 450.6		376.9		$ 883.6	704.5
Net income (loss)				(6.4)	$ (9.3)	13.5	$ (0.8)	(15.7)	12.7
Allocation of the purchase price paid to the fair values of the net assets
Prepaid and other current assets				19.7				19.7		$ 26.3
Property and equipment				878.6				878.6		832.4
Goodwill				258.7				258.7		258.4
Amortization of intangible assets				3.9		$ 6.2		8.2	$ 8.1
Accumulated amortization				54.5				54.5		46.3
Non-compete agreements
Allocation of the purchase price paid to the fair values of the net assets
Accumulated amortization				16.3				16.3		12.8
Customer relationships
Allocation of the purchase price paid to the fair values of the net assets
Accumulated amortization				38.2				$ 38.2		$ 33.5
Minimum
ACQUISITIONS
Number of open-deck trucking companies acquired | item								20
Builders
ACQUISITIONS
Voting interest acquired (as a percent)	100.00%
Total consideration paid	$ 36.3
Cash payments for business consideration	$ 30.0
Number of shares transferred | shares	399,530
Common stock issued in acquisitions	$ 3.4
Outstanding debt assumed	2.9
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable				8.4				$ 8.4
Parts supplies				0.3				0.3
Prepaid and other current assets				1.5				1.5
Property and equipment				29.4				29.4
Goodwill				14.7				14.7
Intangible assets				10.6				10.6
Other long-term assets				0.5				0.5
Deferred tax liability				(9.2)				(9.2)
Accounts payable and other liabilities				(19.9)				(19.9)
Total				36.3				36.3
Transaction expenses incurred, not deductible for taxes purposes	$ 0.2
Aveda
ACQUISITIONS
Total consideration paid			$ 118.7
Cash payments for business consideration			$ 27.3
Number of shares transferred | shares			1,612,979
Common stock issued in acquisitions												$ 15.4
Contingent liability												21.2
Outstanding debt assumed			$ 54.8
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable				37.3				37.3
Prepaid and other current assets				2.5				2.5
Property and equipment				89.8				89.8
Goodwill				7.7				7.7
Intangible assets				15.0				15.0
Deferred tax liability				(6.7)				(6.7)
Accounts payable and other liabilities				(30.0)				(30.0)
Total				115.6				115.6
Cash per common share | $ / shares											$ 0.45
Transaction expenses incurred, not deductible for taxes purposes												$ 1.1
Increase (decrease) in goodwill					0.7
Increase decrease in deferred tax liability					0.7
Kelsey Trail
ACQUISITIONS
Voting interest acquired (as a percent)		100.00%
Total consideration paid		$ 6.2
Cash payments for business consideration		$ 5.3
Number of shares transferred | shares		95,859
Common stock issued in acquisitions		$ 0.9
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable				2.3				2.3
Prepaid and other current assets				0.4				0.4
Property and equipment				9.2				9.2
Goodwill				3.3				3.3
Intangible assets				1.5				1.5
Deferred tax liability				(2.7)				(2.7)
Accounts payable and other liabilities				(8.0)				(8.0)
Total				6.0				6.0
Transaction expenses incurred, not deductible for taxes purposes		$ 0.1
Increase (decrease) in goodwill					(0.9)
Increase decrease in deferred tax liability					$ (0.9)
Leavitts Freight Service [Member]
ACQUISITIONS
Voting interest acquired (as a percent)	100.00%
Total consideration paid	$ 14.9
Allocation of the purchase price paid to the fair values of the net assets
Accounts receivable				1.9				1.9
Parts supplies				0.1				0.1
Prepaid and other current assets				0.4				0.4
Property and equipment				8.5				8.5
Goodwill				5.1				5.1
Intangible assets				3.6				3.6
Accounts payable and other liabilities				(4.9)				(4.9)
Total				$ 14.7				$ 14.7
Transaction expenses incurred, not deductible for taxes purposes	$ 0.3