PENSION AND OTHER SAVINGS PLANS - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Loss due to settlements	$ (2,651,000)	$ 0	$ 0
Defined benefit plan, number of continuous years of employment	5 years
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Term For Loan Covenant Criteria	8 months 15 days
Expected contributions	1,200,000
Total pension contributions	4,800,000
Cash pension contributions	2,900,000
Stock pension contributions	1,900,000
2015 - Pension	8,195,000
2016 - Pension	12,255,000
2017 - Pension	9,235,000
2018 - Pension	9,569,000
2019 - Pension	10,089,000
Years 2020 - 2024 - Pension	56,107,000
401-K [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash pension contributions	6,700,000	1,200,000
Stock pension contributions	1,800,000	2,400,000
Defined Contribution Plan, Cost Recognized	8,500,000	3,600,000	2,900,000
Beulah Mine [Member]
Defined Benefit Plan Disclosure [Line Items]
Loss due to settlements	3,700,000
Loss due to curtailments	$ 900,000