ACQUISITION - Narrative (Details) - USD ($) $ in Thousands	Jan. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Acquisition loan		$ 1,174,650	$ 1,056,809
San Juan Coal Company
Business Acquisition [Line Items]
Cash paid	$ 126,900
Loan from NM Capital Utility Corporation | Term Notes
Business Acquisition [Line Items]
Acquisition loan	$ 125,000	$ 125,000	$ 0