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Income Taxes: (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Effective income tax rate |  |  |
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Change In Tax Rate, Deferred Tax Asset, Provisional Income Tax Expense |  |  | $ 0.0
GOLDEN STATE WATER COMPANY |  |  |
Effective income tax rate |  |  |
ETRs (as a percent) | 19.20% | 36.90% |
Parent [Member] |  |  |
Effective income tax rate |  |  |
ETRs (as a percent) | 19.20% | 35.10% |

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