Electric utility segment - Annual decoupling filings summary (Details) $ in Millions	May 31, 2017 USD ($)
Hawaiian Electric Company
Regulatory Projects and Legal Obligations [Line Items]
2017 Annual incremental RAM adjusted revenues	$ 12.7
Annual change in accrued earnings sharing credits	0.0
Annual change in accrued RBA balance as of December 31, 2016 (and associated revenue taxes) (refunded)	(2.4)
Net annual incremental amount to be collected under the tariffs	10.3
HELCO
Regulatory Projects and Legal Obligations [Line Items]
2017 Annual incremental RAM adjusted revenues	3.2
Annual change in accrued earnings sharing credits	0.0
Annual change in accrued RBA balance as of December 31, 2016 (and associated revenue taxes) (refunded)	(2.5)
Net annual incremental amount to be collected under the tariffs	0.7
Maui Electric
Regulatory Projects and Legal Obligations [Line Items]
2017 Annual incremental RAM adjusted revenues	1.6
Annual change in accrued earnings sharing credits	0.0
Annual change in accrued RBA balance as of December 31, 2016 (and associated revenue taxes) (refunded)	(0.2)
Net annual incremental amount to be collected under the tariffs	$ 1.4