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Income Taxes (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Sep. 30, 2018 | Dec. 31, 2017
Income Taxes |  |
Reduction in net deferred income tax liabilities | $ 11,000 | $ 1,140,000
Regulatory liabilities, non-current | 2,410,597 | 2,452,536
APS |  |
Income Taxes |  |
Reduction in net deferred income tax liabilities |  | 1,140,000
Regulatory liabilities, non-current | 2,410,597 | 2,452,536
Gross-up for revenue requirement of rate regulation |  | 377,000
Excess deferred income taxes - ACC - Tax Cuts and Jobs Act | APS |  |
Income Taxes |  |
Regulatory liabilities, non-current | $ 9,000 | $ 1,520,000

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