Electric utility segment - Rate proceedings (Details) - USD ($) $ in Millions	Apr. 17, 2019	Mar. 18, 2019	Dec. 14, 2018
Maui Electric
Regulatory Projects and Legal Obligations [Line Items]
Interim general rate increase granted	$ 12.2
Effective interest rate of return	7.43%
Stipulated ROACE rate	9.50%
Common equity capitalization rate	57.00%
Rate Base	$ 454.0
Risk sharing percentage, ratepayer		98.00%
Risk sharing percentage, utility		2.00%
Maximum exposure cap		$ 0.6
HELCO
Regulatory Projects and Legal Obligations [Line Items]
Stipulated ROACE rate			10.50%
General rate increase, revenue			$ 13.4
General rate increase, revenue, percent			3.40%
Assumptions on rate of return, return on average common equity, percentage decrease			8.30%