SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Intangible Assets Amortization (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Intangible Assets
Original Cost	$ 60,167	$ 71,020
Accumulated Amortization	(44,018)	(44,378)
Intangibles, Net	16,149	26,642
Noncompete agreement
Intangible Assets
Original Cost	9,697	14,542
Accumulated Amortization	(7,378)	(10,974)
Intangibles, Net	2,319	3,568
Customer contracts and other, net
Intangible Assets
Original Cost	48,970	54,978
Accumulated Amortization	(36,462)	(33,300)
Intangibles, Net	12,508	21,678
Mining Permits
Intangible Assets
Original Cost	1,500	1,500
Accumulated Amortization	(178)	(104)
Intangibles, Net	$ 1,322	$ 1,396

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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