Fair value measurements - Fair Value Hierarchy of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial assets
Available-for-sale investment securities	$ 1,232,826	$ 1,388,533
Held-to-maturity investment securities	143,467	142,057
Financial liabilities
Short-term borrowings—other than bank	185,710	73,992
Other bank borrowings	115,110	110,040
Carrying or notional amount
Financial assets
Available-for-sale investment securities	1,232,826	1,388,533
Held-to-maturity investment securities	139,451	141,875
Stock in Federal Home Loan Bank	8,434	9,958
Loans, net	5,080,107	4,792,707
Mortgage servicing rights	9,101	8,062
Derivative assets	25,179	10,180
Financial liabilities
Deposit liabilities	769,825	827,841
Short-term borrowings—other than bank	185,710	73,992
Other bank borrowings	115,110	110,040
Long-term debt, net—other than bank	1,964,365	1,879,641
Derivative liabilities	51,375	34,132
Carrying or notional amount | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Long-term debt, net—other than bank	1,497,667	1,418,802
Short-term borrowings	88,987	25,000
Estimated fair value
Financial assets
Available-for-sale investment securities	1,232,826	1,388,533
Held-to-maturity investment securities	143,467	142,057
Stock in Federal Home Loan Bank	8,434	9,958
Loans, net	5,157,537	4,802,053
Mortgage servicing rights	12,379	13,618
Derivative assets	300	91
Financial liabilities
Deposit liabilities	765,976	817,667
Short-term borrowings—other than bank	185,710	73,992
Other bank borrowings	115,107	110,037
Long-term debt, net—other than bank	2,156,927	1,904,261
Derivative liabilities	2,218	630
Estimated fair value | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Long-term debt, net—other than bank	1,670,189	1,443,968
Short-term borrowings	88,987	25,000
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—other than bank		0
Derivative liabilities	33	34
Estimated fair value | Level 1 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Long-term debt, net—other than bank	0	0
Short-term borrowings	0	0
Estimated fair value | Level 2
Financial assets
Available-for-sale investment securities	1,204,229	1,364,897
Held-to-maturity investment securities	143,467	142,057
Stock in Federal Home Loan Bank	8,434	9,958
Loans, net	12,295	1,809
Mortgage servicing rights	0	0
Derivative assets	300	91
Financial liabilities
Deposit liabilities	765,976	817,667
Short-term borrowings—other than bank	185,710	73,992
Other bank borrowings	115,107	110,037
Long-term debt, net—other than bank	2,156,927	1,904,261
Derivative liabilities	2,185	596
Estimated fair value | Level 2 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Long-term debt, net—other than bank	1,670,189	1,443,968
Short-term borrowings	88,987	25,000
Estimated fair value | Level 3
Financial assets
Available-for-sale investment securities	28,597	23,636
Held-to-maturity investment securities	0	0
Stock in Federal Home Loan Bank	0	0
Loans, net	5,145,242	4,800,244
Mortgage servicing rights	12,379	13,618
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—other than bank		0
Derivative liabilities	0	0
Estimated fair value | Level 3 | Hawaiian Electric Company, Inc. and Subsidiaries
Financial liabilities
Long-term debt, net—other than bank	0	0
Short-term borrowings	$ 0	$ 0