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Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Changes in accumulated other comprehensive income (loss)        
Beginning Balance     $ 8,028,042 $ 6,373,063
Ending Balance $ 9,257,453 $ 7,645,241 9,257,453 7,645,241
Cash flow hedges, net of tax | Natural gas cash flow hedges, net of tax        
Changes in accumulated other comprehensive income (loss)        
Beginning Balance 52,338 176,370 64,762 217,121
(Gains) losses reclassified from accumulated OCI, net of tax (15,940) (42,581) (28,364) (83,332)
Ending Balance 36,398 133,789 36,398 133,789
Cash flow hedges, net of tax | Interest rate cash flow hedges, net of tax        
Changes in accumulated other comprehensive income (loss)        
Beginning Balance (807) (951) (843) (987)
(Gains) losses reclassified from accumulated OCI, net of tax 36 36 72 72
Ending Balance (771) (915) (771) (915)
Pension and other post- retirement benefits liability adjustment, net of tax        
Changes in accumulated other comprehensive income (loss)        
Beginning Balance (17,328) (16,438) (17,541) (16,640)
(Gains) losses reclassified from accumulated OCI, net of tax 9,622 202 9,835 404
Ending Balance (7,706) (16,236) (7,706) (16,236)
Accumulated OCI, net of tax        
Changes in accumulated other comprehensive income (loss)        
Beginning Balance 34,203 158,981 46,378 199,494
(Gains) losses reclassified from accumulated OCI, net of tax (6,282) (42,343) (18,457) (82,856)
Ending Balance $ 27,921 $ 116,638 $ 27,921 $ 116,638