Electric utility segment - Annual decoupling filings summary (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Hawaiian Electric (parent only)
Regulatory Projects and Legal Obligations [Line Items]
2019 Annual incremental RAM adjusted revenues	$ 14.0
Annual change in accrued RBA balance as of December 31, 2018 (and associated revenue taxes)	(12.2)
2017 Tax Act Adjustment	0.0
Performance Incentive Mechanism	0.1
Net annual incremental amount to be collected under the tariffs	1.9
HELCO
Regulatory Projects and Legal Obligations [Line Items]
2019 Annual incremental RAM adjusted revenues	3.5
Annual change in accrued RBA balance as of December 31, 2018 (and associated revenue taxes)	(1.9)
2017 Tax Act Adjustment	0.0
Performance Incentive Mechanism	0.0
Net annual incremental amount to be collected under the tariffs	1.6
Maui Electric
Regulatory Projects and Legal Obligations [Line Items]
2019 Annual incremental RAM adjusted revenues	3.3
Annual change in accrued RBA balance as of December 31, 2018 (and associated revenue taxes)	0.8
2017 Tax Act Adjustment	2.8	$ (2.8)
Performance Incentive Mechanism	(0.4)
Net annual incremental amount to be collected under the tariffs	$ 6.5