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Credit agreements and long-term debt (Details)
1 Months Ended
Jun. 30, 2018
USD ($)
Oct. 04, 2018
USD ($)
Sep. 30, 2018
USD ($)
Institution
May 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Disclosure [Abstract]          
Number of financial institutions | Institution     8    
Series 2018A | Subsequent Event          
Credit agreement          
Aggregate principal amount   $ 50,000,000      
Fixed coupon interest rate   4.58%      
Debt, face amount   $ 150,000,000      
Series 2018B | Subsequent Event          
Credit agreement          
Aggregate principal amount   $ 100,000,000      
Fixed coupon interest rate   4.72%      
Short-term Borrowings with Bank of Tokyo-Mitsubishi | Subsequent Event          
Credit agreement          
Aggregate principal amount   $ 50,000,000      
Line of credit facility          
Credit agreement          
Revolving noncollateralized credit facility with a letter of credit sub-facility     $ 150,000,000    
Amount outstanding under facilities     0   $ 0
Line of credit facility | Hawaiian Electric Company, Inc. and Subsidiaries          
Credit agreement          
Revolving noncollateralized credit facility with a letter of credit sub-facility     200,000,000    
Amount outstanding under facilities     0   $ 0
Unsecured Debt | Series 2018A          
Credit agreement          
Aggregate principal amount       $ 67,500,000  
Fixed coupon interest rate       4.38%  
Unsecured Debt | Series 2018B          
Credit agreement          
Aggregate principal amount       $ 17,500,000  
Fixed coupon interest rate       4.53%  
Unsecured Debt | Series 2018C          
Credit agreement          
Aggregate principal amount       $ 15,000,000  
Fixed coupon interest rate       4.72%  
Unsecured Debt | Hawaiian Electric Company, Inc. and Subsidiaries | Series 2018A          
Credit agreement          
Aggregate principal amount       $ 52,000,000  
Unsecured Debt | Hawaiian Electric Company, Inc. and Subsidiaries | Series 2018B          
Credit agreement          
Aggregate principal amount       12,500,000  
Unsecured Debt | Hawaiian Electric Company, Inc. and Subsidiaries | Series 2018C          
Credit agreement          
Aggregate principal amount       10,500,000  
Unsecured Debt | Hawaii Electric Light Company, Inc. (HELCO) | Series 2018A          
Credit agreement          
Aggregate principal amount       9,000,000  
Unsecured Debt | Hawaii Electric Light Company, Inc. (HELCO) | Series 2018B          
Credit agreement          
Aggregate principal amount       3,000,000  
Unsecured Debt | Hawaii Electric Light Company, Inc. (HELCO) | Series 2018C          
Credit agreement          
Aggregate principal amount       3,000,000  
Unsecured Debt | Maui Electric | Series 2018A          
Credit agreement          
Aggregate principal amount       6,500,000  
Unsecured Debt | Maui Electric | Series 2018B          
Credit agreement          
Aggregate principal amount       2,000,000  
Unsecured Debt | Maui Electric | Series 2018C          
Credit agreement          
Aggregate principal amount       $ 1,500,000  
Unsecured Debt | Mauo, LLC          
Credit agreement          
Aggregate principal amount     $ 0    
Debt, face amount $ 50,500,000.0        
Unsecured Debt | Mauo, LLC | London Interbank Offered Rate (LIBOR)          
Credit agreement          
Basis spread on variable rate 1.375%