Electric utility segment - Rate proceedings (Details) - USD ($) $ in Millions						1 Months Ended
	Aug. 09, 2018	Jun. 22, 2018	May 01, 2018	Feb. 16, 2018	Aug. 21, 2017	Aug. 31, 2017
Regulatory Projects and Legal Obligations [Line Items]
Interim rate increases				$ 36.0
Rate increase before tax act impact		$ 37.7
Tax act impact on rate, amount		$ 38.3
General rate increase, revenue, calculation assumptions, rate of return, ROACE		9.50%
General rate increase, revenue, calculation assumptions, rate of return		7.57%
Risk sharing percentage, ratepayer		98.00%
Risk sharing percentage, utility		2.00%
Maximum exposure cap		$ 2.5
Maui Electric
Regulatory Projects and Legal Obligations [Line Items]
Interim general rate increase granted	$ 12.5
Effective interest rate of return	7.43%
Stipulated ROACE rate	9.50%
Common equity capitalization rate	57.00%
Rate Base	$ 462.0
HELCO
Regulatory Projects and Legal Obligations [Line Items]
Interim rate increases			$ (9.9)
Interim general rate increase granted					$ 9.9
Stipulated ROACE rate						9.50%