Fair value measurements - Summary of financial assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial assets
Available-for-sale investment securities	$ 1,320,110	$ 1,105,182
Financial liabilities
Short-term borrowings—other than bank	24,498	0
Other bank borrowings	153,552	192,618
American Savings Bank (ASB)
Financial assets
Available-for-sale investment securities	1,320,110	1,105,182
Mortgage-servicing rights	9,070	9,373
Carrying or notional amount
Financial assets
Money market funds		13,085
Available-for-sale investment securities	1,320,110	1,105,182
Stock in Federal Home Loan Bank	9,706	11,218
Loans receivable, net	4,638,962	4,701,977
Mortgage-servicing rights	9,070	9,373
Bank-owned life insurance	147,391	143,197
Derivative assets	8,399	23,578
Financial liabilities
Deposit liabilities	5,752,326	5,548,929
Short-term borrowings—other than bank	24,498
Other bank borrowings	153,552	192,618
Long-term debt, net	1,618,446	1,619,019
Derivative liabilities	500	53,852
Carrying or notional amount | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	8,014
Financial liabilities
Short-term borrowings	6,000
Long-term debt, net	1,318,623	1,319,260
Derivative liabilities		20,734
Estimated fair value
Financial assets
Money market funds		13,085
Available-for-sale investment securities	1,320,110	1,105,182
Stock in Federal Home Loan Bank	9,706	11,218
Loans receivable, net	4,806,937	4,852,826
Mortgage-servicing rights	12,091	13,216
Bank-owned life insurance	147,391	143,197
Derivative assets	591	453
Financial liabilities
Deposit liabilities	5,748,858	5,546,644
Short-term borrowings—other than bank	24,498
Other bank borrowings	153,717	193,991
Long-term debt, net	1,747,972	1,704,717
Derivative liabilities	2	952
Estimated fair value | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	584
Financial liabilities
Short-term borrowings	6,000
Long-term debt, net	1,441,855	1,399,490
Derivative liabilities		743
Estimated fair value | Level 1
Financial assets
Money market funds		0
Available-for-sale investment securities	0	0
Stock in Federal Home Loan Bank	0	0
Loans receivable, net	13,260	0
Mortgage-servicing rights	0	0
Bank-owned life insurance	0	0
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	2	129
Estimated fair value | Level 1 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	0
Financial liabilities
Short-term borrowings	0
Long-term debt, net	0	0
Derivative liabilities		0
Estimated fair value | Level 2
Financial assets
Money market funds		13,085
Available-for-sale investment securities	1,304,683	1,089,755
Stock in Federal Home Loan Bank	9,706	11,218
Loans receivable, net	2,468	13,333
Mortgage-servicing rights	0	0
Bank-owned life insurance	147,391	143,197
Derivative assets	591	453
Financial liabilities
Deposit liabilities	5,748,858	5,546,644
Short-term borrowings—other than bank	24,498
Other bank borrowings	153,717	193,991
Long-term debt, net	1,747,972	1,704,717
Derivative liabilities	0	823
Estimated fair value | Level 2 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	584
Financial liabilities
Short-term borrowings	6,000
Long-term debt, net	1,441,855	1,399,490
Derivative liabilities		743
Estimated fair value | Level 3
Financial assets
Money market funds		0
Available-for-sale investment securities	15,427	15,427
Stock in Federal Home Loan Bank	0	0
Loans receivable, net	4,791,209	4,839,493
Mortgage-servicing rights	12,091	13,216
Bank-owned life insurance	0	0
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 | Level 3 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	0
Financial liabilities
Short-term borrowings	0
Long-term debt, net	$ 0	0
Derivative liabilities		$ 0