COMMITMENTS AND CONTINGENCIES - Leases (Details) $ in Thousands, gal in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) gal	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Future minimum capital lease payments:
2016	$ 35,467
2017	27,695
2018	6,497
2019	4,336
2020	1,297
Thereafter	0
Total minimum lease payments	75,292
Less imputed interest	(4,124)
Present value of minimum capital lease payments	71,168
Future minimum operating lease payments:
2016	12,390
2017	7,409
2018	5,488
2019	4,421
2020	660
Thereafter	2,360
Total	32,728
Rental expense under operating leases	25,200	$ 16,600	$ 11,800
Royalty expense, coal reserves	$ 96,700	61,800	$ 43,600
Minimum annual purchase requirement (in gallons of diesel fuel) | gal	3.9
Property, plant and equipment and mine development assets [Member]
Gross value and accumulated amortization of assets under capital leases:
Gross value	$ 65,293	74,203
Accumulated amortization	25,878	$ 21,598
Other Current Liabilities [Member]
Gross value and accumulated amortization of assets under capital leases:
Remediation obligation	$ 8,100