ACCUMULATED OTHER COMPREHENSIVE INCOME / (LOSS) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 368,391		$ 201,051		$ 662,184		$ 407,833
Crude oil and condensate revenues	70,226		57,466		135,881		107,447
Amortization of net loss	124	[1]	4,174	[1]	249	[1]	8,349	[1]
Income before income taxes	148,035		59,584		218,794		89,328
Tax (expense) / benefit	(58,921)		(23,647)		(86,856)		(35,073)
NET INCOME	89,114		35,937		131,938		54,255
Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Income before income taxes	1,617				16,782
Tax (expense) / benefit	(636)				(6,601)
NET INCOME	981				10,181
Accumulated Net Gains / (Losses) on Cash Flow Hedges | Amount Reclassified from Accumulated Other Comprehensive Income | Commodity contracts
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	(272)				13,056
Crude oil and condensate revenues	2,094				4,136
Accumulated Defined Benefit Pension and Postretirement Plans | Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Amortization of net loss	$ (205)				$ (410)

[1] Net of income taxes of $(81) and $(2,647) for the three months ended June 30, 2013 and 2012, respectively, and $(161) and $(5,294) for the six months ended June 30, 2013 and 2012, respectively.