ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Black Lung	Dec. 31, 2011 Black Lung	Dec. 31, 2010 Black Lung
Accrued Workers Compensation And Pneumoconiosis Benefits
Workers' compensation discount rate (as a percent)	3.22%	3.75%
Letters of credit to secure workers compensation obligation	$ 81.4	$ 69.6
Increase to black lung obligation due to enactment of new legislation					$ 8.3
Prior period before which terminated employees are not included in impact of plan amendment			7 years
Pneumoconiosis discount rate (as a percent)			3.78%	4.25%