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Income taxes - Changes in Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Changes in total unrecognized tax benefits      
Unrecognized tax benefits, at the beginning of the period $ 3.6 $ 0.0 $ 0.9
Reductions based on tax positions taken during the year (0.1) 0.0 0.0
Additions for tax positions of prior years 0.3 3.6 0.1
Settlements (1.0)
Unrecognized tax benefits, at the end of the period 3.8 3.6 0.0
Hawaiian Electric Company, Inc. and Subsidiaries      
Changes in total unrecognized tax benefits      
Unrecognized tax benefits, at the beginning of the period 3.6 0.0 0.5
Reductions based on tax positions taken during the year (0.1) 0.0 0.0
Additions for tax positions of prior years 0.3 3.6 0.1
Settlements 0.0 0.0 (0.6)
Unrecognized tax benefits, at the end of the period $ 3.8 $ 3.6 $ 0.0