SUBSEQUENT EVENTS - Settlement of Litigation (Details) - USD ($) $ in Millions	Feb. 18, 2018	Dec. 31, 2017
Subsequent event | Breach of contract litigation
Settlement of Litigation
Settlement per agreement in principal	$ 93.0
Net amount expected from settlement	80.0
Commitments for external coal purchases
Settlement of Litigation
Purchase of coal reserves, agreed amount		$ 1.3
Commitments for external coal purchases | Subsequent event | Breach of contract litigation
Settlement of Litigation
Purchase of coal reserves, agreed amount	$ 2.0