Income taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense (Benefit) [Line Items]
Deferred tax assets, valuation allowance	$ 0	$ 0
Unrecognized tax benefits that would impact effective tax rate	500,000	500,000
Credit adjustments to interest expense on income taxes	100,000	(100,000)	$ 200,000
Amount of accrued interest related to uncertain tax positions	$ 600,000	400,000
Tax cuts and jobs act, net tax expense			$ 13,400,000
Tax cuts and jobs act, measurement period adjustment, decrease in DTL		13,900,000
Tax cuts and jobs act, measurement period adjustment, income tax benefit		5,500,000
Tax cust and jobs act, increase in regulatory liabilities		$ 11,300,000
Effective income tax rate, before tax act adjustments		22.10%
Effective income tax rate	19.00%	20.00%	39.60%
Hawaiian Electric Company, Inc. and Subsidiaries
Income Tax Expense (Benefit) [Line Items]
Unrecognized tax benefits that would impact effective tax rate	$ 0	$ 0
Credit adjustments to interest expense on income taxes	100,000	100,000	$ 100,000
Amount of accrued interest related to uncertain tax positions	$ 400,000	300,000
Tax cuts and jobs act, net tax expense			$ 9,200,000
Tax cuts and jobs act, measurement period adjustment, decrease in DTL		13,600,000
Tax cuts and jobs act, measurement period adjustment, income tax benefit		$ 5,200,000
Effective income tax rate, before tax act adjustments		22.20%
Effective income tax rate	19.40%	19.30%	40.60%