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Electric utility segment - Annual decoupling filings summary (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Hawaiian Electric (parent only)  
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
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
Maui Electric  
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