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Fair Value of Financial Instruments: (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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GOLDEN STATE WATER COMPANY | Reported Value Measurement |  |
Fair value of financial instruments |  |
Long-term Debt, Fair Value | $ 444,349 | $ 284,699
GOLDEN STATE WATER COMPANY | Estimate of Fair Value Measurement |  |
Fair value of financial instruments |  |
Long-term Debt, Fair Value | 546,363 | $ 376,467
Mutual Funds | Fair Value, Inputs, Level 1 |  |
Financial liabilities: |  |
Long-term debt-GSWC | $ 22,800 |

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