RISK MANAGEMENT (Fair Value of Derivatives) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Assets:
Derivative asset, fair value	$ 74,505	$ 94,865
Derivative asset, liability	(24,105)	(8,255)
Derivative asset, net	14,132	40,771
Long-term derivative asset, net	36,268	45,839
Net recorded fair value	50,400	86,610
Derivative Liabilities:
Derivative liability, fair value	26,848	8,255
Derivative liability, asset	(24,105)	(8,255)
Derivative liability	1,554
Derivative Liabilities Noncurrent	1,189
Net recorded fair value	2,743
Commodity Contracts [Member]
Derivative Assets:
Derivative asset, fair value	74,505
Derivative Liabilities:
Derivative liability, fair value	19,350
Interest Rate Swaps [Member]
Derivative Liabilities:
Derivative liability, fair value	7,498
Derivative Asset [Member]
Derivative Assets:
Derivative asset, fair value	29,553	44,173
Derivative asset, liability	(15,421)	(3,402)
Long-term Derivative Asset [Member]
Derivative Assets:
Derivative asset, fair value	44,952	50,692
Derivative asset, liability	(8,684)	(4,853)
Derivative Liability [Member]
Derivative Liabilities:
Derivative liability, fair value	16,975	3,402
Derivative liability, asset	(15,421)	(3,402)
Long-term Derivative Liability [Member]
Derivative Liabilities:
Derivative liability, fair value	9,873	4,853
Derivative liability, asset	$ (8,684)	$ (4,853)