ACQUISITION - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Jun. 30, 2012 Kemmerer Mine [Member]	Mar. 31, 2012 Kemmerer Mine [Member]	Dec. 31, 2012 Kemmerer Mine [Member]	Jan. 31, 2012 Kemmerer Mine [Member]	Feb. 04, 2011 10.75% Senior Notes [Member]	Jan. 31, 2012 10.75% Senior Notes [Member] Kemmerer Mine [Member]
Business Acquisition [Line Items]
Stated interest rate					10.75%	10.75%
Senior secured note face amount						$ 125.0
Price as a percentage of face value						95.50%
Total purchase consideration				164.5
Cash paid				76.5
Total estimated fair value of liabilities assumed				88.0
Cash bonding obligations financed by the net proceeds of the Add-On Notes				24.7
Acquisition-related costs			1.6
Issuance costs related to the Add-On Notes				5.1
Purchase price allocation of deferred revenue				2.2
Expected increase in revenues and operating income		1.3
Deferred revenue	0.9
Revenue			151.6
Operating income (loss)			$ 27.8