Electric utility segment - Annual Decoupling Filings (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
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 Act Adjustment	0.0
Net annual incremental amount to be collected under the tariffs	4.1
MECO
Regulatory Projects and Legal Obligations [Line Items]
2018 Annual incremental RAM adjusted revenues	2.0
Annual change in accrued RBA balance as of December 31, 2017 (and associated revenue taxes)	3.2
2017 Tax Act Adjustment	(2.8)
Net annual incremental amount to be collected under the tariffs	2.4
Hawaiian Electric
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 Act Adjustment	0.0
Net annual incremental amount to be collected under the tariffs	$ 20.4