CONCENTRATION OF CREDIT RISK AND MAJOR CUSTOMERS (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenue, Major Customer
Trade receivables	$ 153,662	$ 172,724
Customers A and B
Revenue, Major Customer
Trade receivables	45,800	58,900
Illinois Basin | Customer A
Revenue, Major Customer
Total revenues from major customers	319,932	336,560	231,838
Illinois Basin | Customer B
Revenue, Major Customer
Total revenues from major customers	$ 263,582	$ 243,339	$ 249,047