BUSINESS COMBINATIONS - Acquired Intangible Assets and Goodwill (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Amortization expense of intangibles	$ 23.9	$ 23.9	$ 20.7
Goodwill	239.5	239.3	204.7
2018 Acquisitions
Business Acquisition [Line Items]
Amortization expense of intangibles	4.5	2.9
Purchase accounting adjustment to definite-lived and indefinite-lived intangible assets	0.5	0.2
Goodwill		13.6
Goodwill, expected tax deductible amount		13.6
Ready-Mixed Concrete Segment
Business Acquisition [Line Items]
Purchase accounting adjustment to definite-lived and indefinite-lived intangible assets		0.8	1.3
Goodwill	$ 150.0	149.8	$ 136.7
Ready-Mixed Concrete Segment | 2018 Acquisitions
Business Acquisition [Line Items]
Goodwill		12.8
Non-Reportable Segments | 2018 Acquisitions
Business Acquisition [Line Items]
Goodwill		$ 0.8