Related party transactions (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Hawaiian Electric Company, Inc. and Subsidiaries
Related Party Transaction [Line Items]
General management and administrative services from related parties	$ 1.4	$ 2.1
Hawaii Medical Service Association (HMSA) | HMSA costs
Related Party Transaction [Line Items]
Expenses from transactions with related party	7.0	7.0
Hawaii Medical Service Association (HMSA) | HMSA expense
Related Party Transaction [Line Items]
Expenses from transactions with related party	5.0	5.0
Hawaii Dental Service (HDS) | HDS costs
Related Party Transaction [Line Items]
Expenses from transactions with related party	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
Hawaiian Electric Company, Inc. and Subsidiaries | Hawaii Medical Service Association (HMSA) | HMSA costs
Related Party Transaction [Line Items]
Expenses from transactions with related party	6.0	6.0
Hawaiian Electric Company, Inc. and Subsidiaries | Hawaii Medical Service Association (HMSA) | HMSA expense
Related Party Transaction [Line Items]
Expenses from transactions with related party	4.0	3.0
Hawaiian Electric Company, Inc. and Subsidiaries | Hawaii Dental Service (HDS) | HDS costs
Related Party Transaction [Line Items]
Expenses from transactions with related party	1.0	1.0
Hawaiian Electric Company, Inc. and Subsidiaries | Hawaii Dental Service (HDS) | HDS expense
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ 0.0	$ 0.0