Electric utility segment - Annual decoupling filings summary (Details) $ in Millions	Mar. 29, 2018 USD ($)
Hawaiian Electric Company
Regulatory Projects and Legal Obligations [Line Items]
2018 Annual incremental RAM adjusted revenues	$ 13.8
Annual change in accrued RBA balance as of December 31, 2017 (and associated revenue taxes)	6.6
2017 Tax Reform Act Adjustment	0.0
Net annual incremental amount to be collected under the tariffs	20.4
HELCO
Regulatory Projects and Legal Obligations [Line Items]
2018 Annual incremental RAM adjusted revenues	3.4
Annual change in accrued RBA balance as of December 31, 2017 (and associated revenue taxes)	0.7
2017 Tax Reform Act Adjustment	0.0
Net annual incremental amount to be collected under the tariffs	4.1
Maui Electric
Regulatory Projects and Legal Obligations [Line Items]
2018 Annual incremental RAM adjusted revenues	2.3
Annual change in accrued RBA balance as of December 31, 2017 (and associated revenue taxes)	3.2
2017 Tax Reform Act Adjustment	(2.4)
Net annual incremental amount to be collected under the tariffs	$ 3.1