COMMITMENTS AND CONTINGENCIES - Purchase Commitments (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Commitments for external coal purchases
Contractual Commitments
Contractual amount due in 2018	$ 1.3
Commitments related to planned capital projects
Contractual Commitments
Contractual amount	$ 64.3

Minimum amount of purchase arrangement maturing in the next fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

No definition available.

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