Basis of Presentation Changes in Accumulated Other Comprehensive Income / (Loss) by component (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Natural Gas Swaps	$ (38,379)	$ (77,492)	$ (93,146)	$ (251,357)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(649,896)		(649,896)		(747,342)
Other Comprehensive Income (Loss), Net of Tax	13,872	(46,347)	97,446	1,763
Tax Benefit of Natural Gas Swaps	14,025	29,683	36,551	97,760
Natural Gas Swaps, Net of Tax	(24,354)	(47,809)	(56,595)	(153,597)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(8,212)	(13,915)	(24,635)	(39,937)
Defined Benefit Plan, Amortization of Gains (Losses)	21,055	26,611	69,802	79,688
Other Comprehensive Income (Loss), Finalization of Pension and Non-Pension Postretirement Plan Valuation, Net of Tax	(6,296)		38,498
Defined Benefit Plan, Net Periodic Benefit Cost	19,139	12,696	83,665	39,751
Tax Expense on Actuarially Determined Long-Term Liabilities	7,306	4,775	(31,936)	(14,946)
Actuarially Determined Long-Term Liability Adjustments, Net of Tax	11,833	7,921	51,729	24,805
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(649,896)		(649,896)		(747,342)
Other Comprehensive Income (Loss), Net of Tax			97,446
Amounts reclassified from accumulated other comprehensive income			(4,866)
Other comprehensive income before reclassifications			102,312
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	60,566		60,566		76,761
Other Comprehensive Income (Loss), Net of Tax			(16,195)
Amounts reclassified from accumulated other comprehensive income			(56,595)
Other comprehensive income before reclassifications			40,400
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(710,462)		(710,462)		(824,103)
Other Comprehensive Income (Loss), Net of Tax			113,641
Amounts reclassified from accumulated other comprehensive income			51,729
Other comprehensive income before reclassifications			61,912
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(408)	(408)	(1,223)	(1,223)
Defined Benefit Plan, Amortization of Gains (Losses)	(8,042)	(11,959)	(30,764)	(35,876)
Other Comprehensive Income (Loss), Finalization of Pension and Non-Pension Postretirement Plan Valuation, Net of Tax	(6,296)	0	(38,498)	0
Defined Benefit Plan, Net Periodic Benefit Cost	$ 14,988	$ 14,936	$ 73,281	$ 43,756