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Summary of significant accounting policies - Accounts Receivable and Additional Information (Details) - USD ($)
$ in Thousands
 12 Months Ended
Aug. 11, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounts Receivable        
Allowance for customer accounts receivable, accrued unbilled revenues and other accounts receivable   $ 1,200 $ 1,100  
Electric utility revenues        
Weighted average AFUDC rate 1.75%      
Goodwill and other intangibles        
Goodwill   82,190 82,190  
American Savings Bank (ASB)        
Goodwill and other intangibles        
Goodwill   82,190 82,190  
Affordable housing investment, carrying value   59,016 47,081  
Commitments to fund affordable housing investments   $ 15,800 14,000  
American Savings Bank (ASB) | Loans receivable        
Nonperforming loans        
Period for classification as delinquent   90 days    
American Savings Bank (ASB) | Mortgage receivable        
Nonperforming loans        
Period for classification as delinquent   180 days    
American Savings Bank (ASB) | Consumer        
Nonperforming loans        
Period for write-off   120 days    
Hawaiian Electric Company, Inc. and Subsidiaries        
Electric utility revenues        
Revenue taxes included in revenues and in taxes, other than income taxes expense   $ 202,000 187,000 $ 209,000
Revenue taxes accrued   $ 115,000 $ 104,000  
Weighted average AFUDC rate   7.70% 7.60% 7.60%
Hawaiian Electric Company, Inc. and Subsidiaries | Minimum        
Contributions in aid of construction        
Contributions amortized period   30 years    
Hawaiian Electric Company, Inc. and Subsidiaries | Maximum        
Contributions in aid of construction        
Contributions amortized period   55 years    
American Savings Bank (ASB)        
Loans modified in a troubled debt restructuring        
Number of consecutive months of repayment required for loans to be removed from nonaccrual status   6 months    
Goodwill and other intangibles        
Goodwill   $ 82,200 $ 82,200