ACCUMULATED OTHER COMPREHENSIVE INCOME / (LOSS) (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Natural gas revenues	$ 293,793		$ 206,782
Crude oil and condensate revenues	65,655		49,981
Amortization of net loss	125	[1]	4,175	[1]
Income before income taxes	70,759		29,744
Tax (expense) / benefit	(27,935)		(11,426)
NET INCOME	42,824		18,318
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	15,165
Tax (expense) / benefit	(5,965)
NET INCOME	9,200
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	13,328
Crude oil and condensate revenues	2,042
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)

[1] Net of income taxes of $(80) and $(2,647) for the three months ended March 31, 2013 and 2012, respectively.