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Income taxes - Additional Information (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Expense (Benefit) [Line Items]        
Deferred tax assets, valuation allowance $ 0 $ 0 $ 0  
Unrecognized tax benefits that would impact effective tax rate 500,000 500,000 500,000  
Credit adjustments to interest expense on income taxes   (100,000) 200,000 $ 200,000
Amount of accrued interest related to uncertain tax positions 500,000 400,000 500,000  
Decrease in net deferred tax liability   13,900,000 271,500,000  
Net tax expense due to impact of the Tax Act 14,200,000   13,400,000  
Tax Act, Increase in regulatory liabilities   11,300,000 $ 284,900,000  
Effect of remeasurement of deferred tax assets and liabilities   $ 5,500,000    
Effective income tax rate before adjustments   22.10%    
Effective income tax rate   20.00% 39.60% 33.10%
Hawaiian Electric Company, Inc. and Subsidiaries        
Income Tax Expense (Benefit) [Line Items]        
Credit adjustments to interest expense on income taxes   $ 100,000 $ 100,000 $ 30,000
Amount of accrued interest related to uncertain tax positions 200,000 300,000 200,000  
Decrease in net deferred tax liability   13,600,000 275,700,000  
Net tax expense due to impact of the Tax Act $ 9,200,000   $ 9,200,000  
Effect of remeasurement of deferred tax assets and liabilities   $ 5,200,000    
Effective income tax rate before adjustments   22.20%    
Effective income tax rate   19.30% 40.60% 37.00%