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Summary of Significant Accounting Policies - Advances for Construction and Contributions in Aid of Construction (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019
Accounting Policies [Abstract] |  |
Refund period for advances for construction received after 1981 | 40 years |
Estimated Refunds of Advances for Construction [Abstract] |  |
2021 | $ 3,050 |
2022 | 3,050 |
2023 | 3,038 |
2024 | 2,958 |
2025 | 2,896 |
Thereafter | 52,462 |
Advances for construction | 125,027 | $ 112,339
Customer advances for construction, non-refundable | 29,117 |
Customer advances for construction, refunded on service connections | $ 28,456 |
Contract with customer, refund period | 40 years |
Contract with customer, fair value of advances | $ 50,391 | $ 50,035
Amortization period for taxes paid relating to advances and contributions | 40 years |

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