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Summary of significant accounting policies (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Equity method      
Composite annual depreciation rate (as a percent) 3.20% 3.10% 3.10%
Leases      
Operating lease expense $ 18 $ 19 $ 19
Future minimum lease payments      
2016 11    
2017 10    
2018 7    
2019 6    
2020 4    
Thereafter 10    
Total $ 48    
Maximum      
Cash and cash equivalents      
Original maturity period of investments classified as cash equivalents 3 months    
Equity method      
Ownership percentage of affiliates classified as equity method investments 50.00%    
Hawaiian Electric Company, Inc. and Subsidiaries      
Leases      
Operating lease expense $ 9 $ 9 $ 8
Future minimum lease payments      
2016 5    
2017 4    
2018 3    
2019 2    
2020 2    
Thereafter 6    
Total $ 22