COMMITMENTS AND CONTINGENCIES - Reclamation Bonds (Details) - USD ($) $ in Millions		6 Months Ended
Jun. 23, 2017	Jun. 30, 2017
Reclamation Bonds
Surety bonds upfront deposit amount		$ 0.5
Outstanding surety bonds		$ 4.9
Reclamation Bonds
Reclamation Bonds
Face amount	$ 20.0
Percentage of annual fees on surety bonds	2.00%
Surety bonds upfront deposit amount	$ 0.0

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant