Fair value measurements - Fair Value Hierarchy of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Financial assets
Available-for-sale investment securities	$ 2,574,618	$ 1,970,417
Held-to-maturity investment securities	510,474
Financial liabilities
Short-term borrowings—other than bank	53,998	129,379
Other bank borrowings	88,305	89,670
Carrying or notional amount
Financial assets
Available-for-sale investment securities	2,574,618	1,970,417
Held-to-maturity investment securities	522,270	226,947
Loans, net	5,150,388	5,260,917
Mortgage servicing rights	9,950	10,020
Derivative assets	57,377	120,980
Financial liabilities
Deposit liabilities	423,976	548,830
Short-term borrowings—other than bank	53,998	129,379
Other bank borrowings	88,305	89,670
Long-term debt, net	2,321,937	2,119,129
Derivative liabilities	57,000	137,500
Carrying or notional amount | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		49,979
Long-term debt, net	1,676,402	1,561,302
Estimated fair value
Financial assets
Available-for-sale investment securities	2,574,618	1,970,417
Held-to-maturity investment securities	510,474	229,963
Loans, net	5,228,524	5,439,330
Mortgage servicing rights	14,480	10,705
Derivative assets	909	4,536
Financial liabilities
Deposit liabilities	442,361	552,800
Short-term borrowings—other than bank	53,998	129,379
Other bank borrowings	88,304	89,669
Long-term debt, net	2,624,130	2,487,790
Derivative liabilities	5,282	5,030
Estimated fair value | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		49,979
Long-term debt, net	1,955,710	1,890,490
Estimated fair value | 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	0	0
Financial liabilities
Deposit liabilities	0	0
Short-term borrowings—other than bank	0	0
Other bank borrowings	0	0
Long-term debt, net	0	0
Derivative liabilities	11	500
Estimated fair value | Level 1 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		0
Long-term debt, net	0	0
Estimated fair value | Level 2
Financial assets
Available-for-sale investment securities	2,559,191	1,943,232
Held-to-maturity investment securities	510,474	229,963
Loans, net	10,403	28,354
Mortgage servicing rights	0	0
Derivative assets	909	4,536
Financial liabilities
Deposit liabilities	442,361	552,800
Short-term borrowings—other than bank	53,998	129,379
Other bank borrowings	88,304	89,669
Long-term debt, net	2,624,130	2,487,790
Derivative liabilities	5,271	4,530
Estimated fair value | Level 2 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		49,979
Long-term debt, net	1,955,710	1,890,490
Estimated fair value | Level 3
Financial assets
Available-for-sale investment securities	15,427	27,185
Held-to-maturity investment securities	0	0
Loans, net	5,218,121	5,410,976
Mortgage servicing rights	14,480	10,705
Derivative assets	0	0
Financial liabilities
Deposit liabilities	0	0
Short-term borrowings—other than bank	0	0
Other bank borrowings	0	0
Long-term debt, net	0	0
Derivative liabilities	0	0
Estimated fair value | Level 3 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Short-term borrowings		0
Long-term debt, net	$ 0	$ 0