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Capitalization (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Nov. 30, 2020 | Aug. 31, 2019 | Feb. 28, 2019 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | May 31, 2020 | Sep. 30, 2018 | Aug. 31, 2018
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Other long term debt |  |  |  | $ 129.4 |  | $ 129.6 |  |  |
Customer advances for construction |  |  |  | $ 23.3 | $ 23.4 |  |  |  |
First Mortgage Bonds [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity, construction loan |  | $ 53.7 |  |  |  |  |  |  |
Series 2019A [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity, construction loan |  | $ 32.5 |  |  |  |  |  |  |
Interest rate |  | 4.00% |  |  |  |  |  |  |
Series 2019B [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity, construction loan |  | $ 21.2 |  |  |  |  |  |  |
Interest rate |  | 5.00% |  |  |  |  |  |  |
New Jersey Economic Development Authority [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Debt Instrument Maturity Date |  |  | Dec. 31, 2022 |  |  |  |  |  |
Proceeds from issuance of first mortgage bond |  |  | $ 140.0 |  |  |  |  |  |
New Jersey NJIB Program [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity, construction loan |  |  |  |  |  |  |  | $ 8.7 |
New Jersey Infrastructure Bank [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity, construction loan |  |  |  |  |  |  |  |  | $ 43.5
Percentage of principal with stated interest rate |  |  |  | 0.00% |  |  |  |  |
Percentage of principal with market interest rate |  |  |  | 0.00% |  |  |  |  |
Middlesex [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing amount |  |  |  | $ 42.6 |  |  |  |  |
Proceeds from private placement | $ 40.0 |  |  |  |  |  |  |  |
Middlesex [Member] | New Jersey Board Of Public Utilities [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing amount |  |  |  |  |  |  | $ 100.0 |  |
FMB [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Percentage of principal with stated interest rate | 2.90% |  |  |  |  |  |  |  |
Debt Instrument Maturity Date | Nov. 30, 2050 |  |  |  |  |  |  |  |
First Mortgage Bonds [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from issuance premium |  | $ 7.1 |  |  |  |  |  |  |
Middlesex Water Company Investment Plan [Member] |  |  |  |  |  |  |  |  |
Schedule of Capitalization [Line Items] |  |  |  |  |  |  |  |  |
Proceeds amount |  |  |  | $ 0.3 | $ 0.3 |  |  |  |
Issuance of common stock |  |  |  | 5,000 | 5,000 |  |  |  |

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