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Capitalization (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 6 Months Ended
Nov. 30, 2020
Aug. 31, 2019
Feb. 28, 2019
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
May 31, 2020
Sep. 30, 2018
Aug. 31, 2018
Schedule of Capitalization [Line Items]                  
Other long term debt       $ 129.3   $ 129.6      
Customer advances for construction       $ 23.6 $ 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]                  
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       $ 0.6 $ 0.6        
Issuance of common stock       7,787 9,378