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Electric utility segment - Unconsolidated Variable Interest Entities (Details)
 1 Months Ended 12 Months Ended
Mar. 31, 2004
USD ($)
security
Dec. 31, 2017
USD ($)
agreement
entity
Variable Interest Entity [Line Items]    
Number of power purchase agreements (PPAs) | agreement   5
Hawaiian Electric Company, Inc (HECO)    
Variable Interest Entity [Line Items]    
Number of power purchase agreements (PPAs) | agreement   5
Number of entities owning wind farms not required to be consolidated as VIE's | entity   2
Minimum potential number of IPP entities consolidated into company in the future | entity   1
Variable Interest Entity, Not Primary Beneficiary    
Variable Interest Entity [Line Items]    
Investment in 2004 Debentures   $ 51,500,000
Interest income   3,400,000
Variable Interest Entity, Not Primary Beneficiary | Hawaiian Electric Company, Inc (HECO)    
Variable Interest Entity [Line Items]    
Principal amount of 2004 Debentures $ 31,500,000.0  
Variable Interest Entity, Not Primary Beneficiary | Hawaii Electric Light Company, Inc. (HELCO)    
Variable Interest Entity [Line Items]    
Principal amount of 2004 Debentures $ 10,000,000  
Variable Interest Entity, Not Primary Beneficiary | 2004 Trust Preferred Securities | Hawaiian Electric Company, Inc (HECO)    
Variable Interest Entity [Line Items]    
Number of 2004 Trust Preferred Securities issued | security 2,000,000  
Dividend rate on 2004 Trust Preferred Securities 6.50%  
Aggregate Liquidation preference $ 50,000,000  
Balance of Trust Securities   50,000,000
Dividend distributions on Trust Preferred Securities   3,300,000
Variable Interest Entity, Not Primary Beneficiary | Trust Common Securities | Hawaiian Electric Company, Inc (HECO)    
Variable Interest Entity [Line Items]    
Aggregate Liquidation preference $ 1,500,000.0  
Balance of Trust Securities   1,500,000
Common dividend   $ 100,000