Fair value measurements - Summary of financial assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial assets
Available-for-sale investment securities	$ 1,418,490	$ 1,401,198
Held-to-maturity investment securities	42,491	44,412
Financial liabilities
Short-term borrowings—other than bank	238,445	117,945
Other bank borrowings	100,430	190,859
Carrying or notional amount
Financial assets
Available-for-sale investment securities	1,418,490	1,401,198
Held-to-maturity investment securities	43,450	44,515
Stock in Federal Home Loan Bank	10,158	9,706
Loans, net	4,695,508	4,628,381
Mortgage-servicing rights	8,541	8,639
Derivative assets	29,840	17,812
Financial liabilities
Deposit liabilities	777,390	5,890,597
Short-term borrowings—other than bank	238,445	117,945
Other bank borrowings	100,430	190,859
Long-term debt, net	1,684,002	1,683,797
Derivative liabilities	24,985	13,562
Carrying or notional amount | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	3,240	3,240
Financial liabilities
Long-term debt, net	1,368,627	1,368,479
Short-term borrowings	121,983	4,999
Estimated fair value
Financial assets
Available-for-sale investment securities	1,418,490	1,401,198
Held-to-maturity investment securities	42,491	44,412
Stock in Federal Home Loan Bank	10,158	9,706
Loans, net	4,780,250	4,781,751
Mortgage-servicing rights	12,882	12,052
Derivative assets	611	393
Financial liabilities
Deposit liabilities	766,425	5,884,071
Short-term borrowings—other than bank	238,445	117,945
Other bank borrowings	100,377	190,829
Long-term debt, net	1,741,324	1,813,295
Derivative liabilities	87	30
Estimated fair value | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	329	256
Financial liabilities
Long-term debt, net	1,432,134	1,497,079
Short-term borrowings	121,983	4,999
Estimated fair value | Level 1
Financial assets
Available-for-sale investment securities	0	0
Held-to-maturity investment securities	0	0
Stock in Federal Home Loan Bank	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	60	20
Estimated fair value | Level 1 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	0	0
Financial liabilities
Long-term debt, net	0	0
Short-term borrowings	0	0
Estimated fair value | Level 2
Financial assets
Available-for-sale investment securities	1,403,063	1,385,771
Held-to-maturity investment securities	42,491	44,412
Stock in Federal Home Loan Bank	10,158	9,706
Loans, net	7,380	11,254
Mortgage-servicing rights	0	0
Derivative assets	611	393
Financial liabilities
Deposit liabilities	766,425	5,884,071
Short-term borrowings—other than bank	238,445	117,945
Other bank borrowings	100,377	190,829
Long-term debt, net	1,741,324	1,813,295
Derivative liabilities	27	10
Estimated fair value | Level 2 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial assets
Derivative assets	329	256
Financial liabilities
Long-term debt, net	1,432,134	1,497,079
Short-term borrowings	121,983	4,999
Estimated fair value | Level 3
Financial assets
Available-for-sale investment securities	15,427	15,427
Held-to-maturity investment securities	0	0
Stock in Federal Home Loan Bank	0	0
Loans, net	4,772,870	4,770,497
Mortgage-servicing rights	12,882	12,052
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 assets
Derivative assets	0	0
Financial liabilities
Long-term debt, net	0	0
Short-term borrowings	$ 0	$ 0