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Electric utility segment - Rate proceedings (Details) - USD ($) $ in Millions								1 Months Ended
	Mar. 18, 2019	Dec. 14, 2018	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]
Risk sharing percentage, ratepayer	98.00%
Risk sharing percentage, utility	2.00%
Maximum exposure cap	$ 0.6
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
HELCO
Regulatory Projects and Legal Obligations [Line Items]
Interim rate increases					$ (9.9)
Interim general rate increase granted							$ 9.9
Stipulated ROACE rate		10.50%						9.50%
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%

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