COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Undelivered feedstock	$ 9.2	$ 12.4
Shortfall fee expenses	1.1	0.6	$ 0.5
Mine closure and environmental	$ 0.9	$ 0.9	$ 0.7

The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109237650&loc=d3e13064-110858

Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13296-110859

Name:	us-gaap_EnvironmentalRemediationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration