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Fair Value of Financial Instruments: (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
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GOLDEN STATE WATER COMPANY | Reported Value Measurement |  |
Fair value of financial instruments |  |
Long-term Debt, Fair Value | $ 444,172 | $ 444,271
GOLDEN STATE WATER COMPANY | Estimate of Fair Value Measurement |  |
Fair value of financial instruments |  |
Long-term Debt, Fair Value | 519,888 | $ 559,752
Mutual Funds | Fair Value, Inputs, Level 1 |  |
Financial liabilities: |  |
Long-term debt-GSWC | $ 26,500 |

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