COMMITMENTS AND CONTINGENCIES - Leases (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 gal	Dec. 31, 2013	Dec. 31, 2012
Future minimum capital lease payments:
2015	$ 41,927,000
2016	38,997,000
2017	28,273,000
2018	5,313,000
2019	2,346,000
Thereafter	198,000
Total minimum lease payments	117,054,000
Less imputed interest	(7,703,000)
Present value of minimum capital lease payments	109,351,000
Future minimum operating lease payments:
2015	16,886,000
2016	10,098,000
2017	6,554,000
2018	5,072,000
2019	4,231,000
Thereafter	3,942,000
Total	46,783,000
Rental expense under operating leases	16,600,000	11,800,000	9,500,000
Royalty expense, coal reserves	61,800,000	43,630,000	40,100,000
Minimum annual purchase requirement (in gallons of diesel fuel)	4,500,000
Property, plant and equipment and mine development assets [Member]
Gross value and accumulated amortization of assets under capital leases:
Gross value	74,203,000	24,982,000
Accumulated amortization	$ 21,598,000	$ 11,983,000