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Share-based compensation - Related Income Tax Benefits (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense (in dollars) $ 5,400,000 $ 4,800,000 $ 6,500,000
Income tax benefit 1,900,000 1,600,000 2,300,000
Capitalized share-based compensation expense 0 0 150,000
Hawaiian Electric Company, Inc. and Subsidiaries      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense (in dollars) 1,900,000 1,400,000 1,900,000
Income tax benefit $ 700,000 $ 500,000 700,000
Capitalized share-based compensation expense     $ 0