Basis of Presentation Changes in Accumulated Other Comprehensive Income / (Loss) by component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Natural Gas Swaps	$ (34,697)	$ (79,321)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(742,893)		(747,342)
Other Comprehensive Income (Loss), Net of Tax	4,449	87,708
Tax Benefit of Natural Gas Swaps	11,984	31,380
Natural Gas Swaps, Net of Tax	(22,713)	(47,941)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(8,212)	(12,106)
Defined Benefit Plan, Amortization of Gains (Losses)	25,188	27,005
Other Comprehensive Income (Loss), Finalization of Pension and Non-Pension Postretirement Plan Valuation, Net of Tax	(27,115)
Defined Benefit Plan, Net Periodic Benefit Cost	44,091	14,899
Tax Expense on Actuarially Determined Long-Term Liabilities	16,831	5,602
Actuarially Determined Long-Term Liability Adjustments, Net of Tax	27,260	9,297
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(742,893)		(747,342)
Other Comprehensive Income (Loss), Net of Tax	4,449
Amounts reclassified from accumulated other comprehensive income	4,547
Other comprehensive income before reclassifications	(98)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	35,453		76,761
Other Comprehensive Income (Loss), Net of Tax	(41,308)
Amounts reclassified from accumulated other comprehensive income	(22,713)
Other comprehensive income before reclassifications	(18,595)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(778,346)		(824,103)
Other Comprehensive Income (Loss), Net of Tax	45,757
Amounts reclassified from accumulated other comprehensive income	27,260
Other comprehensive income before reclassifications	18,497
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(408)	(408)
Defined Benefit Plan, Amortization of Gains (Losses)	(12,175)	(12,263)
Other Comprehensive Income (Loss), Finalization of Pension and Non-Pension Postretirement Plan Valuation, Net of Tax	(27,115)	0
Defined Benefit Plan, Net Periodic Benefit Cost	$ 41,287	$ 14,759