Electric utility segment - Annual decoupling filings summary (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Regulatory Projects and Legal Obligations [Line Items]
2022 ARA revenues	$ 29.5
Management Audit savings commitment	(6.6)
Net 2022 ARA revenues	22.9
Incremental RAM revenues and ARA revenues	61.5
Incremental accrued RBA balance through September 30, 2021 (and associated revenue taxes)	24.3
Incremental Performance Incentive Mechanisms (net)	0.1
Incremental MPIR/EPRM Revenue Adjustment	10.4
Net incremental amount to be collected under the RBA rate tariffs	96.2
Hawaiian Electric, Parent
Regulatory Projects and Legal Obligations [Line Items]
2022 ARA revenues	19.8
Management Audit savings commitment	(4.6)
Net 2022 ARA revenues	15.2
Incremental RAM revenues and ARA revenues	41.7
Incremental accrued RBA balance through September 30, 2021 (and associated revenue taxes)	21.9
Incremental Performance Incentive Mechanisms (net)	0.0
Incremental MPIR/EPRM Revenue Adjustment	9.8
Net incremental amount to be collected under the RBA rate tariffs	73.4
Hawaii Electric Light
Regulatory Projects and Legal Obligations [Line Items]
2022 ARA revenues	4.9
Management Audit savings commitment	(1.0)
Net 2022 ARA revenues	3.9
Incremental RAM revenues and ARA revenues	8.9
Incremental accrued RBA balance through September 30, 2021 (and associated revenue taxes)	2.5
Incremental Performance Incentive Mechanisms (net)	0.0
Incremental MPIR/EPRM Revenue Adjustment	0.3
Net incremental amount to be collected under the RBA rate tariffs	11.7
Maui Electric
Regulatory Projects and Legal Obligations [Line Items]
2022 ARA revenues	4.8
Management Audit savings commitment	(1.0)
Net 2022 ARA revenues	3.8
Incremental RAM revenues and ARA revenues	10.9
Incremental accrued RBA balance through September 30, 2021 (and associated revenue taxes)	(0.1)
Incremental Performance Incentive Mechanisms (net)	0.1
Incremental MPIR/EPRM Revenue Adjustment	0.3
Net incremental amount to be collected under the RBA rate tariffs	$ 11.1