DERIVATIVE INSTRUMENTS (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current assets
Derivative contracts not designated as hedging instruments and their balance sheet location
Commodity derivative asset	$ 28,690	$ 79,487
Interest rate derivative asset	11,500	10,134
Total assets	40,190	89,621
Noncurrent assets
Derivative contracts not designated as hedging instruments and their balance sheet location
Commodity derivative asset	6,775	0
Interest rate derivative asset	1,560	10,422
Total assets	8,335	10,422
Current liabilities
Derivative contracts not designated as hedging instruments and their balance sheet location
Commodity derivative liability	9,347	28,944
Noncurrent liabilities
Derivative contracts not designated as hedging instruments and their balance sheet location
Commodity derivative liability	$ 7,204	$ 0