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Capitalization (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended
Nov. 30, 2021
Nov. 30, 2020
Aug. 31, 2019
Sep. 30, 2021
Sep. 30, 2020
Oct. 31, 2021
Jun. 30, 2021
Dec. 31, 2020
May 31, 2020
Sep. 30, 2018
Aug. 31, 2018
Schedule of Capitalization [Line Items]                      
Other long term debt       $ 148.4       $ 129.6      
Customer advances for construction       $ 23.5 $ 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 NJIB Program [Member]                      
Schedule of Capitalization [Line Items]                      
Maximum borrowing capacity, construction loan                   $ 8.7  
Delaware SRF Program [Member] | Subsequent Event [Member]                      
Schedule of Capitalization [Line Items]                      
Maximum borrowing capacity, construction loan           $ 5.0          
Private Placement Loan [Member] | Subsequent Event [Member]                      
Schedule of Capitalization [Line Items]                      
Percentage of principal with stated interest rate 2.90%                    
Proceeds from private placement $ 45.5                    
Debt Instrument Maturity Date Nov. 30, 2051                    
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%              
FMB [Member]                      
Schedule of Capitalization [Line Items]                      
Percentage of principal with stated interest rate   2.90%                  
Proceeds from private placement   $ 40.0   $ 59.5              
Debt Instrument Maturity Date   Nov. 30, 2050                  
Amount of maximum redeem martgage bonds             $ 45.5        
FMB [Member] | Series RR [Member]                      
Schedule of Capitalization [Line Items]                      
Amount of maximum redeem martgage bonds             22.5        
FMB [Member] | Series SS [Member]                      
Schedule of Capitalization [Line Items]                      
Amount of maximum redeem martgage bonds             $ 23.0        
FMB [Member] | Subsequent Event [Member]                      
Schedule of Capitalization [Line Items]                      
Percentage of principal with stated interest rate 2.79%                    
Proceeds from private placement $ 19.5                    
Debt Instrument Maturity Date Nov. 30, 2041                    
Tidewater [Member]                      
Schedule of Capitalization [Line Items]                      
Other long term debt       $ 20.0              
Percentage of principal with stated interest rate       3.94%              
Debt Instrument Maturity Date       Sep. 30, 2046              
Middlesex [Member] | New Jersey Board Of Public Utilities [Member]                      
Schedule of Capitalization [Line Items]                      
Maximum borrowing amount                 $ 100.0    
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       $ 0.9 $ 0.9            
Issuance of common stock       10,641 14,223            
Percentage of discount purchase of common stock in investment plan       3.00%              
Number of shares offering for discount       200,000