INCOME TAX Income Tax Disclosure (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Business Acquisition [Line Items]
Decrease to deferred tax assets		$ (46.1)
Release of valuation allowance	$ 46.1
San Juan Coal Company
Business Acquisition [Line Items]
Deferred income taxes		$ 46.1