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Electric utility segment - Utility Projects (Details)
 1 Months Ended 12 Months Ended
Jun. 10, 2019
USD ($)
Aug. 11, 2016
USD ($)
Dec. 31, 2020
USD ($)
MWh
Nov. 30, 2019
USD ($)
MW
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Regulatory Projects and Legal Obligations [Line Items]            
Project costs, amortization     $ 1,300,000   $ 1,300,000  
Federal            
Regulatory Projects and Legal Obligations [Line Items]            
Tax credit deferral period         25 years  
State            
Regulatory Projects and Legal Obligations [Line Items]            
Tax credit deferral period         10 years  
Hawaiian Electric Company, Inc. and Subsidiaries            
Regulatory Projects and Legal Obligations [Line Items]            
Regulatory liabilities     37,301,000   $ 37,301,000 $ 30,724,000
Hawaiian Electric Company, Inc. and Subsidiaries | ERP/EAM Implementation Project            
Regulatory Projects and Legal Obligations [Line Items]            
ERP/EAM cost recovery cap   $ 77,600,000        
ERP/EAM required pass-through savings   $ 246,000,000        
ERP/EAM project service period (in years) 12 years 12 years     12 years  
Weighted average AFUDC rate   1.75%        
ERP/EAM implementation project costs, accrued carrying costs once put into service     58,800,000   $ 58,800,000  
ERP/EAM implementation project, expected future O&M expense reductions $ 150,000,000          
ERP/EAM implementation project, future cost avoidance related to capital costs and tax costs $ 96,000,000          
ERP/EAM regulatory liability for operation and maintenance expense reductions     10,800,000   10,800,000  
Hawaiian Electric Company, Inc. and Subsidiaries | West Loch PV Project            
Regulatory Projects and Legal Obligations [Line Items]            
Solar project, energy production (in megawatts) | MW       20    
Solar project, project cap       $ 67,000,000    
Solar project, project cap for in-kind work       $ 4,700,000    
Solar project, cost incurred     53,300,000   53,300,000  
Federal nonrefundable tax credits     14,700,000   14,700,000  
State nonrefundable tax credits     $ 14,000,000.0   $ 14,000,000.0  
Solar project, maximum energy costs (in dollars per KWH)         9.56  
Annual production target (megawatt per hour) | MWh     46,850      
Regulatory liabilities     $ 600,000   $ 600,000  
Hawaiian Electric | ERP/EAM Implementation Project            
Regulatory Projects and Legal Obligations [Line Items]            
ERP/EAM regulatory liability for operation and maintenance expense reductions     6,900,000   6,900,000  
HELCO | ERP/EAM Implementation Project            
Regulatory Projects and Legal Obligations [Line Items]            
ERP/EAM regulatory liability for operation and maintenance expense reductions     1,600,000   1,600,000  
Maui Electric | ERP/EAM Implementation Project            
Regulatory Projects and Legal Obligations [Line Items]            
ERP/EAM regulatory liability for operation and maintenance expense reductions     $ 2,300,000   $ 2,300,000  
Regulatory liability, amortization period         5 years