ACQUISITIONS AND DIVESTITURES (Narrative) (Details)		3 Months Ended		12 Months Ended
		Mar. 31, 2019 USD ($) item	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Significant Acquisitions and Disposals [Line Items]
Total consideration				$ 219,863,000
Cash consideration		$ (1,122,000)	$ 76,259,000	215,363,000
Consideration payable amount				4,500,000
Number of facilities acquired | item		0
Number of facilities divested | item		2
Gain on sale of property, plant & equipment and businesses		$ 7,297,000	4,164,000
Assets held for sale		0	0	0
Goodwill decrease	[1]	(3,554,000)
Acquisitions 2018 [Member]
Significant Acquisitions and Disposals [Line Items]
Amortizable intangible assets recognized				$ 44,163,000
Intangible assets amortization period, tax purposes				15 years
Goodwill				$ 42,325,000
Intangible assets, deductible for income tax purposes				7,385,000
Goodwill, deductible for income tax purposes				4,468,000
Deferred income taxes, net				32,871,000
Georgia [Member]
Significant Acquisitions and Disposals [Line Items]
Gain on sale of property, plant & equipment and businesses		$ 4,064,000	$ 2,929,000
Contractual Rights In Place - Straight-Line Method [Member] | Acquisitions 2018 [Member]
Significant Acquisitions and Disposals [Line Items]
Amortizable intangible assets recognized				$ 43,072,000
Estimated weighted-average amortization period of intangible assets				19 years 10 months 24 days
Contractual Rights In Place - Units Of Sales [Member] | Acquisitions 2018 [Member]
Significant Acquisitions and Disposals [Line Items]
Amortizable intangible assets recognized				$ 1,080,000
Estimated weighted-average amortization period of intangible assets				30 years

[1] See Note 16 for a summary of prior year acquisitions.