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New Accounting Standards (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2018 | Jul. 03, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |
Pre-tax income | $ 404,218 | $ 425,264 | $ 626,698 | $ 718,944 |  |
Reclassification of income tax effect related to tax reform |  |  | (8,552) |  |  |
APS |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |
Pre-tax income | 428,772 | $ 435,663 | 673,824 | $ 734,851 |  |
Reclassification of income tax effect related to tax reform |  |  | (5,038) |  |  |
Accounting Standards Update 2017-07 |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |
Pre-tax income | $ 4,000 |  | 11,000 |  |  |
Accounting Standards Update 2017-07 | Scenario, Forecast |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |
Net periodic benefit cost (credit) |  |  |  |  | $ 15,000 |
Accounting Standards Update 2018-02 |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |
Reclassification of income tax effect related to tax reform |  |  | 9,000 |  |  |
Accounting Standards Update 2018-02 | APS |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |
Reclassification of income tax effect related to tax reform |  |  | $ 5,000 |  |  |
Four Corners | APS |  |  |  |  |  |
Error Corrections and Prior Period Adjustments Restatement [Line Items] |  |  |  |  |  |
Ownership interest (as a percent) | 7.00% |  | 7.00% |  |  | 7.00%

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