Fair value measurements - Carrying or Notional Amount, Fair Value and Placement in the Fair Value Hierarchy of the Company’s Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Financial assets
Available-for-sale investment securities	$ 1,266,412	$ 1,429,667
Held-to-maturity investment securities	1,052,221
Financial liabilities
Short-term borrowings—other than bank	0	172,568
Other bank borrowings	750,000	695,120
Carrying or notional amount
Financial assets
Available-for-sale investment securities	1,266,412	1,429,667
Held-to-maturity investment securities	1,212,005	1,251,747
Loans, net	6,116,811	5,907,514
Mortgage servicing rights	8,376	9,047
Derivative assets	19,044	16,220
Financial liabilities
Deposit liabilities	1,050,692	611,718
Short-term borrowings—other than bank		172,568
Other bank borrowings	750,000	695,120
Long-term debt, net	2,944,589	2,384,980
Derivative liabilities	22,949	22,949
Carrying or notional amount | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		87,967
Long-term debt, net	2,034,040	1,684,816
Estimated fair value
Financial assets
Available-for-sale investment securities	1,266,412	1,429,667
Held-to-maturity investment securities	1,052,221	1,150,971
Loans, net	5,543,986	5,454,202
Mortgage servicing rights	18,867	17,646
Derivative assets	1,365	1,348
Financial liabilities
Deposit liabilities	1,035,501	597,617
Short-term borrowings—other than bank		172,568
Other bank borrowings	743,366	695,095
Long-term debt, net	2,120,573	2,122,605
Derivative liabilities	1,344	472
Estimated fair value | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		87,967
Long-term debt, net	1,400,901	1,487,496
Estimated fair value | Quoted prices in active markets for identical assets (Level 1)
Financial assets
Available-for-sale investment securities	0	0
Held-to-maturity investment securities	0	0
Loans, net	0	0
Mortgage servicing rights	0	0
Derivative assets	15	18
Financial liabilities
Deposit liabilities	0	0
Short-term borrowings—other than bank		0
Other bank borrowings	0	0
Long-term debt, net	0	0
Derivative liabilities	0	0
Estimated fair value | Quoted prices in active markets for identical assets (Level 1) | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		0
Long-term debt, net	0	0
Estimated fair value | Significant  other observable  inputs (Level 2)
Financial assets
Available-for-sale investment securities	1,251,918	1,414,765
Held-to-maturity investment securities	1,052,221	1,150,971
Loans, net	2,174	821
Mortgage servicing rights	0	0
Derivative assets	1,350	1,330
Financial liabilities
Deposit liabilities	1,035,501	597,617
Short-term borrowings—other than bank		172,568
Other bank borrowings	743,366	695,095
Long-term debt, net	2,120,573	2,122,605
Derivative liabilities	1,344	472
Estimated fair value | Significant  other observable  inputs (Level 2) | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		87,967
Long-term debt, net	1,400,901	1,487,496
Estimated fair value | Significant unobservable inputs (Level 3)
Financial assets
Available-for-sale investment securities	14,494	14,902
Held-to-maturity investment securities	0	0
Loans, net	5,541,812	5,453,381
Mortgage servicing rights	18,867	17,646
Derivative assets	0	0
Financial liabilities
Deposit liabilities	0	0
Short-term borrowings—other than bank		0
Other bank borrowings	0	0
Long-term debt, net	0	0
Derivative liabilities	0	0
Estimated fair value | Significant unobservable inputs (Level 3) | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		0
Long-term debt, net	$ 0	$ 0