COMMITMENTS AND CONTINGENCIES - Leases (Details) m³ in Thousands, $ in Thousands, gal in Millions		12 Months Ended
	Oct. 31, 2013 m³	Dec. 31, 2016 USD ($) gal	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Gross value and accumulated amortization of assets under capital leases:
Water released in containment pond breach, cubic meters | m³	670
Future minimum capital lease payments:
2017		$ 35,345
2018		10,467
2019		6,666
2020		3,237
2021		1,772
Thereafter		1,644
Total minimum lease payments		59,131
Less imputed interest		(4,070)
Present value of minimum capital lease payments		55,061
Future minimum operating lease payments:
2017		9,574
2018		7,784
2019		5,763
2020		1,350
2021		1,312
Thereafter		5,100
Total		30,883
Rental expense under operating leases		18,200	$ 25,200	$ 16,600
Royalty expense, coal reserves		$ 99,200	96,700	$ 61,800
Minimum annual purchase requirement (in gallons of diesel fuel) | gal		9.4
Property, plant and equipment and mine development assets [Member]
Gross value and accumulated amortization of assets under capital leases:
Gross value		$ 74,335	65,293
Accumulated amortization		32,335	$ 25,878
Other Current Liabilities [Member]
Gross value and accumulated amortization of assets under capital leases:
Remediation obligation		$ 5,800