Electric utility segment - Schedule of Annual Decoupling Filings Summary (Details) - USD ($) $ in Millions		6 Months Ended
	Mar. 31, 2026	Mar. 31, 2026
Regulatory Projects and Legal Obligations [Line Items]
2026 ARA revenues	$ 30.3
Management Audit savings commitment	(6.6)
Recovery of COVID-19 related costs	2.7
Return of Enterprise Resource Planning benefit liability	(3.3)
Net 2026 ARA revenues	23.1
Incremental ARA revenues		$ (0.6)
Incremental PIMs (net)		1.1
Incremental MPIR/EPRM revenue adjustment		3.6
Incremental Pilot Process cost recovery		(0.9)
Net incremental amount to be collected under the RBA rate tariffs		3.2
Hawaiian Electric
Regulatory Projects and Legal Obligations [Line Items]
2026 ARA revenues	20.4
Management Audit savings commitment	(4.6)
Recovery of COVID-19 related costs	1.8
Return of Enterprise Resource Planning benefit liability	0.0
Net 2026 ARA revenues	17.6
Incremental ARA revenues		(0.4)
Incremental PIMs (net)		(2.5)
Incremental MPIR/EPRM revenue adjustment		0.7
Incremental Pilot Process cost recovery		(0.6)
Net incremental amount to be collected under the RBA rate tariffs		(2.8)
Hawaii Electric Light
Regulatory Projects and Legal Obligations [Line Items]
2026 ARA revenues	5.0
Management Audit savings commitment	(1.0)
Recovery of COVID-19 related costs	0.4
Return of Enterprise Resource Planning benefit liability	(1.3)
Net 2026 ARA revenues	3.1
Incremental ARA revenues		(0.1)
Incremental PIMs (net)		2.2
Incremental MPIR/EPRM revenue adjustment		1.1
Incremental Pilot Process cost recovery		(0.2)
Net incremental amount to be collected under the RBA rate tariffs		3.0
Maui Electric
Regulatory Projects and Legal Obligations [Line Items]
2026 ARA revenues	4.9
Management Audit savings commitment	(1.0)
Recovery of COVID-19 related costs	0.5
Return of Enterprise Resource Planning benefit liability	(2.0)
Net 2026 ARA revenues	$ 2.4
Incremental ARA revenues		(0.1)
Incremental PIMs (net)		1.4
Incremental MPIR/EPRM revenue adjustment		1.8
Incremental Pilot Process cost recovery		(0.1)
Net incremental amount to be collected under the RBA rate tariffs		$ 3.0