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Summary of significant accounting policies - Leases and Additional Information (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2019
Leases        
Operating lease expense $ 21 $ 20 $ 19  
Future minimum lease payments        
2019 11      
2020 9      
2021 8      
2022 5      
2023 4      
Thereafter 12      
Total 49      
ASU 2016-02 | Scenario, Forecast        
Future minimum lease payments        
Operating lease, liability       $ 257
Operating lease, right-of-use asset       257
ASU 2016-02 | Scenario, Forecast | PPAs        
Future minimum lease payments        
Operating lease, liability       215
Operating lease, right-of-use asset       215
Hawaiian Electric Company, Inc. and Subsidiaries        
Leases        
Operating lease expense 11 $ 11 $ 10  
Future minimum lease payments        
2019 6      
2020 6      
2021 5      
2022 2      
2023 2      
Thereafter 3      
Total $ 24      
Hawaiian Electric Company, Inc. and Subsidiaries | ASU 2016-02 | Scenario, Forecast        
Future minimum lease payments        
Operating lease, liability       236
Operating lease, right-of-use asset       $ 236