ACCUMULATED OTHER COMPREHENSIVE INCOME / (LOSS) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 341,901		$ 231,896		$ 1,004,085		$ 639,729
Crude oil and condensate revenues	84,209		57,870		220,090		165,317
Amortization of net loss	70	[1]	79	[1]	319	[1]	8,428	[1]
Income before income taxes	115,736		59,556		334,530		148,885
Tax (expense) / benefit	(45,847)		(22,948)		(132,703)		(58,021)
NET INCOME	69,889		36,608		201,827		90,864
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	19,568				36,351
Tax (expense) / benefit	(7,696)				(14,298)
NET INCOME	11,872				22,053
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	20,766				33,822
Crude oil and condensate revenues	(1,082)				3,054
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	$ (116)				$ (525)

[1] Net of income taxes of $(46) and $(53) for the three months ended September 30, 2013 and 2012, respectively, and $(206) and $(5,347) for the nine months ended September 30, 2013 and 2012, respectively.