Electric utility segment - Related-Party Transactions (Details) - Hawaiian Electric Industries, Inc. - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Hamakua Energy Partners, L.P. (HEP)
Related Party Transaction [Line Items]
Amount charged to subsidiaries for general management and administrative services	$ 3.0	$ 56.0
Hawaiian Electric Company, Inc (HECO)
Related Party Transaction [Line Items]
Amount charged to subsidiaries for general management and administrative services		$ 5.9	$ 6.2	$ 6.5
Effective interest rate basis term		30 days
Line of credit facility basis point spread		0.15%