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Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period |  | $ (43,626) | $ 5,135,730 | $ 4,935,912
OCI (loss) before reclassifications |  | 9 | (6,024) | (2,911)
Amounts reclassified from accumulated other comprehensive loss | $ 1,550 | 1,500 | 4,504 | 4,616
Reclassification of income tax effect related to tax reform |  |  | (8,552) |
Balance at end of period | 5,485,861 | 5,277,607 | 5,485,861 | 5,277,607
Pension and Other Postretirement Benefits |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period | (54,233) | (39,881) | (42,440) | (39,070)
OCI (loss) before reclassifications |  | 0 | (5,928) | (2,157)
Amounts reclassified from accumulated other comprehensive loss | 1,099 | 790 | 3,188 | 2,136
Reclassification of income tax effect related to tax reform |  |  | (7,954) |
Balance at end of period | (53,134) | (39,091) | (53,134) | (39,091)
Derivative Instruments |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period | (2,391) | (3,745) | (2,562) | (4,752)
OCI (loss) before reclassifications |  | 9 | (96) | (754)
Amounts reclassified from accumulated other comprehensive loss | 451 | 710 | 1,316 | 2,480
Reclassification of income tax effect related to tax reform |  |  | (598) |
Balance at end of period | (1,940) | (3,026) | (1,940) | (3,026)
Accumulated Other Comprehensive Income (Loss) |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period | (56,624) |  | (45,002) | (43,822)
Reclassification of income tax effect related to tax reform |  |  | (8,552) |
Balance at end of period | (55,074) | (42,117) | (55,074) | (42,117)
APS |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period |  |  | 5,385,869 | 5,037,970
OCI (loss) before reclassifications |  | 9 | (5,887) | (2,875)
Amounts reclassified from accumulated other comprehensive loss | 1,403 | 1,487 | 4,152 | 4,682
Reclassification of income tax effect related to tax reform |  |  | (5,038) |
Balance at end of period | 5,757,571 | 5,373,351 | 5,757,571 | 5,373,351
APS | Pension and Other Postretirement Benefits |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period | (32,768) | (21,367) | (24,421) | (20,671)
OCI (loss) before reclassifications |  | 0 | (5,791) | (2,121)
Amounts reclassified from accumulated other comprehensive loss | 952 | 777 | 2,836 | 2,202
Reclassification of income tax effect related to tax reform |  |  | (4,440) |
Balance at end of period | (31,816) | (20,590) | (31,816) | (20,590)
APS | Derivative Instruments |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period | (2,391) | (3,745) | (2,562) | (4,752)
OCI (loss) before reclassifications |  | 9 | (96) | (754)
Amounts reclassified from accumulated other comprehensive loss | 451 | 710 | 1,316 | 2,480
Reclassification of income tax effect related to tax reform |  |  | (598) |
Balance at end of period | (1,940) | (3,026) | (1,940) | (3,026)
APS | Accumulated Other Comprehensive Income (Loss) |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Balance at beginning of period | (35,159) | (25,112) | (26,983) | (25,423)
Reclassification of income tax effect related to tax reform |  |  | (5,038) |
Balance at end of period | $ (33,756) | $ (23,616) | $ (33,756) | $ (23,616)

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