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	$ 126.9
Assets:
Inventories - coal and supplies	8.8
Other current assets	9.5
Contractual third-party reclamation receivable	4.6
Total current assets	22.9
Land and mineral rights	108.8
Plant and equipment	73.5
Contractual third-party reclamation receivable	66.8
Other assets	10.5
Total Assets	282.5
Liabilities:
Trade payables and other accrued liabilities	13.5
Production taxes	2.0
Other current liabilities	9.4
Asset retirement obligations	4.6
Total current liabilities	29.5
Asset retirement obligations, less current portion	66.8
Postretirement medical	1.9
Other liabilities	9.4
Deferred tax liabilities	48.0
Total Liabilities	155.6
Net fair value	$ 126.9