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Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2017 | Mar. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax |  |
Beginning Balance | $ 9,119,247 | $ 8,028,042
(Gains) losses reclassified from accumulated OCI, net of tax | (776) | (12,175)
Ending Balance | 9,310,576 | 8,481,571
Cash flow hedges, net of tax | Natural gas cash flow hedges, net of tax |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax |  |
Beginning Balance | 9,607 | 64,762
(Gains) losses reclassified from accumulated OCI, net of tax | 888 | 12,424
Ending Balance | 8,719 | 52,338
Cash flow hedges, net of tax | Interest rate cash flow hedges, net of tax |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax |  |
Beginning Balance | (699) | (843)
(Gains) losses reclassified from accumulated OCI, net of tax | (36) | (36)
Ending Balance | (663) | (807)
Other post- retirement benefit liability adjustment, net of tax |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax |  |
Beginning Balance | (6,866) | (17,541)
(Gains) losses reclassified from accumulated OCI, net of tax | 76 | 213
Ending Balance | (6,790) | (17,328)
Accumulated OCI, net of tax |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax |  |
Beginning Balance | 2,042 | 46,378
Ending Balance | $ 1,266 | $ 34,203

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