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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

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- Definition Amount after tax of reclassification adjustments of other comprehensive income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 55 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=66004921&loc=d3e1436-108581 + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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