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Summary of Significant Accounting Policies - Cash and Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands |  | 12 Months Ended |
--- | --- | --- | ---
Dec. 19, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 23, 2020
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Long-term borrowings held as restricted cash |  | $ 4,000 | $ 5,000 | $ 0 |
Long-term debt |  | 1,352,320 | 1,292,586 |  |
Maine Water Company |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Long-term borrowings held as restricted cash | $ 5,000 |  |  |  |
Long-term debt |  | $ 39,751 | $ 38,032 |  |
Maine Water Company | State Revolving Fund Loans Series T | Public utility, bonds |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Long-term debt |  |  |  |  | $ 4,000

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