XML 151 R123.htm IDEA: XBRL DOCUMENT v3.3.1.900
Income taxes Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Expense (Benefit) [Line Items]        
Reconciliation percentage of amount of income taxes computed at federal statutory rate 35.00% 35.00% 35.00%  
Out-of-period tax benefit related to utilities     $ 2,700,000  
Credit adjustments to interest expense on income taxes   $ 1,700,000 300,000  
Amount of accrued interest related to uncertain tax positions $ 100,000 0    
Unrecognized tax benefits 3,600,000 0 900,000 $ 800,000
Out-of-period income tax benefit     3,100,000  
Expected increase in depreciation expense, amount $ 117,000,000      
Research tax credit, percent of cost of qualified research expenses 20.00%      
Renewable energy investment tax credit, percent 30.00%      
Hawaiian Electric Company, Inc. and Subsidiaries        
Income Tax Expense (Benefit) [Line Items]        
Credit adjustments to interest expense on income taxes   700,000 300,000  
Amount of accrued interest related to uncertain tax positions $ 100,000 100,000    
Unrecognized tax benefits $ 3,600,000 $ 0 $ 500,000 $ 400,000