SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Investments, Stockpiles, Proven and Probable Reserves (Details) $ in Millions	Jan. 01, 2018 USD ($)
Marketable equity securities \| Accounting Standards Update 2016-01
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 3.0

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

No definition available.

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