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Income taxes - Income Tax Reconciliation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Expense (Benefit) [Line Items]      
Amount at the federal statutory income tax rate $ 96,796 $ 130,844 $ 89,176
Increase (decrease) resulting from:      
State income taxes, net of federal income tax benefit 9,789 13,915 8,097
Net deferred tax asset adjustment related to the Tax Act 13,420 0 0
Other, net (10,612) (21,064) (4,252)
Federal and state taxes $ 109,393 $ 123,695 $ 93,021
Effective income tax rate 39.60% 33.10% 36.50%
Hawaiian Electric Company, Inc. and Subsidiaries      
Income Tax Expense (Benefit) [Line Items]      
Amount at the federal statutory income tax rate $ 71,801 $ 80,190 $ 75,996
Increase (decrease) resulting from:      
State income taxes, net of federal income tax benefit 7,584 8,494 6,726
Net deferred tax asset adjustment related to the Tax Act 9,168 0 0
Other, net (5,354) (3,883) (3,300)
Federal and state taxes $ 83,199 $ 84,801 $ 79,422
Effective income tax rate 40.60% 37.00% 36.60%