Related party transactions (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Hawaiian Electric Company, Inc. and Subsidiaries
Related Party Transaction [Line Items]
General management and administrative services from related parties	$ 2.3	$ 1.5	$ 4.4	$ 3.2
American Savings Bank (ASB)
Related Party Transaction [Line Items]
General management and administrative services from related parties	0.6	0.6	1.4	1.2
Hawaii Medical Service Association (HMSA) | HMSA costs
Related Party Transaction [Line Items]
Expenses from transactions with related party	7.0	8.0	14.0	15.0
Hawaii Medical Service Association (HMSA) | HMSA costs | Hawaiian Electric Company, Inc. and Subsidiaries
Related Party Transaction [Line Items]
Expenses from transactions with related party	6.0	6.0	11.0	11.0
Hawaii Medical Service Association (HMSA) | HMSA expense
Related Party Transaction [Line Items]
Expenses from transactions with related party	5.0	5.0	10.0	11.0
Hawaii Medical Service Association (HMSA) | HMSA expense | Hawaiian Electric Company, Inc. and Subsidiaries
Related Party Transaction [Line Items]
Expenses from transactions with related party	3.0	4.0	7.0	7.0
Hawaii Dental Service (HDS) | HDS costs
Related Party Transaction [Line Items]
Expenses from transactions with related party	1.0	1.0	1.0	2.0
Hawaii Dental Service (HDS) | HDS costs | Hawaiian Electric Company, Inc. and Subsidiaries
Related Party Transaction [Line Items]
Expenses from transactions with related party	1.0	1.0	1.0	1.0
Hawaii Dental Service (HDS) | HDS expense
Related Party Transaction [Line Items]
Expenses from transactions with related party	1.0	1.0	1.0	1.0
Hawaii Dental Service (HDS) | HDS expense | Hawaiian Electric Company, Inc. and Subsidiaries
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ 0.0	$ 0.0	$ 1.0	$ 1.0