ACQUISITION - Summary of purchase consideration and allocation of purchase consideration (Details) - San Juan Coal Company - USD ($) $ in Millions		12 Months Ended
	Jan. 31, 2016	Dec. 31, 2016
Purchase Price:
Cash paid	$ 121.0	$ 121.0
Assets:
Inventories		8.8
Total current assets		8.8
Land and mineral rights		143.9
Plant and equipment		74.6
Other assets		1.3
Total Assets		228.6
Liabilities:
Trade payables and other accrued liabilities		13.4
Production taxes		2.0
Asset retirement obligations		0.7
Total current liabilities		16.1
Asset retirement obligations, less current portion		43.5
Postretirement medical benefits		1.9
Deferred income taxes		46.1
Total Liabilities		107.6
Net fair value		$ 121.0