v2.4.0.8
Basis of Presentation Changes in Accumulated Other Comprehensive Income / (Loss) by component (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Natural Gas Swaps $ (20,070) $ (94,544) $ (54,767) $ (173,865)  
Accumulated Other Comprehensive Income (Loss), Net of Tax (663,768)   (663,768)   (747,342)
Other Comprehensive Income (Loss), Net of Tax 79,125 (39,598) 83,574 48,110  
Tax Benefit of Natural Gas Swaps 10,542 36,697 22,526 68,077  
Natural Gas Swaps, Net of Tax (9,528) (57,847) (32,241) (105,788)  
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (8,211) (13,915) (16,423) (26,021)  
Defined Benefit Plan, Amortization of Gains (Losses) 23,559 26,072 48,747 53,077  
Other Comprehensive Income (Loss), Finalization of Pension and Non-Pension Postretirement Plan Valuation, Net of Tax (5,087)   32,202    
Defined Benefit Plan, Net Periodic Benefit Cost 20,435 12,157 64,526 27,056  
Tax Expense on Actuarially Determined Long-Term Liabilities 7,800 4,571 (24,631) (10,173)  
Actuarially Determined Long-Term Liability Adjustments, Net of Tax 12,635 7,586 39,895 16,883  
Accumulated Other Comprehensive Income (Loss)
          
Accumulated Other Comprehensive Income (Loss), Net of Tax (663,768)   (663,768)   (747,342)
Other Comprehensive Income (Loss), Net of Tax     83,574    
Amounts reclassified from accumulated other comprehensive income     7,654    
Other comprehensive income before reclassifications     75,920    
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
          
Accumulated Other Comprehensive Income (Loss), Net of Tax 71,674   71,674   76,761
Other Comprehensive Income (Loss), Net of Tax     (5,087)    
Amounts reclassified from accumulated other comprehensive income     (32,241)    
Other comprehensive income before reclassifications     27,154    
Accumulated Defined Benefit Plans Adjustment [Member]
          
Accumulated Other Comprehensive Income (Loss), Net of Tax (735,442)   (735,442)   (824,103)
Other Comprehensive Income (Loss), Net of Tax     88,661    
Amounts reclassified from accumulated other comprehensive income     39,895    
Other comprehensive income before reclassifications     48,766    
Pension Plans, Defined Benefit [Member]
          
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (407) (407) (815) (815)  
Defined Benefit Plan, Amortization of Gains (Losses) (10,547) (11,654) (22,722) (23,917)  
Other Comprehensive Income (Loss), Finalization of Pension and Non-Pension Postretirement Plan Valuation, Net of Tax (5,087) 0 (32,202) 0  
Defined Benefit Plan, Net Periodic Benefit Cost $ 17,006 $ 14,060 $ 58,293 $ 28,819