ACQUISITION - Summary of purchase consideration and allocation of purchase consideration (Details) - San Juan Coal Company $ in Millions	Jan. 31, 2016 USD ($)
Purchase Price:
Cash paid	$ 125.3
Assets:
Inventories - coal and supplies	8.8
Other receivables	10.0
Contractual third-party reclamation receivable	4.6
Total current assets	23.4
Land and mineral rights	108.3
Plant and equipment	73.5
Contractual third-party reclamation receivable	66.8
Other assets	10.4
Total Assets	282.4
Liabilities:
Trade payables and other accrued liabilities	14.1
Production taxes	2.0
Other liabilities	9.9
Asset retirement obligations	4.6
Total current liabilities	30.6
Asset retirement obligations, less current portion	66.8
Postretirement medical	1.9
Deferred income taxes	48.5
Other liabilities	9.3
Total Liabilities	157.1
Net fair value	$ 125.3