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Summary of significant accounting policies - Leases and Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Leases      
Operating lease expense $ 20,000 $ 19,000 $ 18,000
Future minimum lease payments      
2018 11,000    
2019 10,000    
2020 8,000    
2021 7,000    
2022 4,000    
Thereafter 36,000    
Total 76,000    
Reclass of AOCI for tax rate reduction impact 0    
Retained earnings      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact 7,440    
Retained earnings | Accounting Standards Update 2018-02      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact 7,400    
AOCI      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact (7,440)    
AOCI | Accounting Standards Update 2018-02      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact (7,400)    
Hawaiian Electric Company, Inc. and Subsidiaries      
Leases      
Operating lease expense 11,000 $ 10,000 $ 9,000
Future minimum lease payments      
2018 6,000    
2019 5,000    
2020 5,000    
2021 5,000    
2022 3,000    
Thereafter 29,000    
Total 53,000    
Reclass of AOCI for tax rate reduction impact 0    
Hawaiian Electric Company, Inc. and Subsidiaries | Retained earnings      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact 209    
Hawaiian Electric Company, Inc. and Subsidiaries | Retained earnings | Accounting Standards Update 2018-02      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact 200    
Hawaiian Electric Company, Inc. and Subsidiaries | AOCI      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact (209)    
Hawaiian Electric Company, Inc. and Subsidiaries | AOCI | Accounting Standards Update 2018-02      
Future minimum lease payments      
Reclass of AOCI for tax rate reduction impact $ (200)