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Electric utility segment - Commitments and contingencies (Details)
 1 Months Ended 3 Months Ended 9 Months Ended
Nov. 13, 2015
MW
Oct. 30, 2015
USD ($)
Oct. 26, 2015
USD ($)
Aug. 03, 2015
state
May. 28, 2015
USD ($)
kWh
Apr. 15, 2015
USD ($)
Mar. 31, 2015
USD ($)
May. 31, 2013
Feb. 16, 2012
generation_unit
Sep. 30, 2015
USD ($)
Jul. 31, 2015
USD ($)
Jun. 30, 2015
MW
Aug. 31, 2014
Jul. 31, 2014
USD ($)
Apr. 30, 2014
order
Aug. 31, 2012
MW
Mar. 31, 1988
MW
Sep. 30, 2015
USD ($)
Sep. 30, 2015
USD ($)
Sep. 30, 2014
USD ($)
Dec. 31, 2014
USD ($)
Jun. 20, 2014
facility
Mar. 01, 2014
Feb. 07, 2014
Regulatory projects and legal obligations                                                
Purchase commitment, period                                 30 years              
Minimum power volume required | MW                               186 180              
Additional capacity requirement | MW                       9                        
Enterprise resource management system project, estimated costs                           $ 82,400,000                    
Enterprise resource management system project, difference between Option A and B                     $ 20,800,000                          
SAP software costs                                   $ 4,800,000            
Schofield generating station facility capacity | MW                               50                
Schofield generating station project, budgetary cap                   $ 167,000,000                            
Percent of costs recoverable through recovery mechanisms other than base rates                   90.00%                            
Environmental regulation                                                
Number of EGUs impacted by proposed rules of MATS | generation_unit                 14                              
Period for which operations was stopped prior to merger                                     4 years          
States included in interim state-wide emission limits | state       48                                        
Public Utilities, Phase-in Plans [Abstract]                                                
Annual incremental RAM adjusted revenues         $ 11,100,000 $ 26,200,000 $ 31,600,000                                  
Net annual incremental amount to be collected under the tariffs         400,000 $ 14,700,000                                    
Number of orders from regulatory agency | order                             4                  
Percent of energy production from renewable energy sources                         65.00%                      
Subsequent Event                                                
Regulatory projects and legal obligations                                                
Additional capacity requirement | MW 9                                              
Changes in the asset retirement obligation liability                                                
Revenue requirement associated with 2015 Plant additions     $ 40,300,000                                          
Reserve for Environmental Costs [Member]                                                
Environmental regulation                                                
Valuation allowances and reserves                                         $ 800,000      
Hawaiian Electric Company, Inc. and Subsidiaries                                                
Environmental regulation                                                
Percentage of reduction in GHG emissions by 2020                                           16.00%    
Number of facilities affected by GHG rule | facility                                           11    
Estimated annual fee for greenhouse gas emissions                                     $ 500,000          
Impact on earnings from recognition of AROs                                     0          
Changes in the asset retirement obligation liability                                                
Balance, beginning of period                                     29,419,000 $ 43,106,000        
Accretion expense                                     18,000 816,000        
Liabilities incurred                                     0 0        
Liabilities settled                                     (2,349,000) (11,338,000)        
Revisions in estimated cash flows                                     0 0        
Balance, end of period                   $ 27,088,000               27,088,000 27,088,000 $ 32,584,000        
Decoupling implementation experience, period               3 years                                
Proposed rate base adjustment, percent of previous rate base adjustment                                               90.00%
Effective interest rate, revenue balancing account                                               6.00%
Public Utilities, Phase-in Plans [Abstract]                                                
Period to file required plan                             120 days                  
Hawaiian Electric Company, Inc. and Subsidiaries | Minimum                                                
Changes in the asset retirement obligation liability                                                
Proposed effective interest rate, revenue balancing account                                             1.25%  
Hawaiian Electric Company, Inc. and Subsidiaries | Maximum                                                
Changes in the asset retirement obligation liability                                                
Proposed effective interest rate, revenue balancing account                                             3.25%  
Hawaiian Electric Company                                                
Environmental regulation                                                
Period of extension resulting in MATS compliance date                 1 year                              
Public Utilities, Phase-in Plans [Abstract]                                                
Annual incremental RAM adjusted revenues         8,100,000                                      
Annual change in accrued earnings sharing credits to be refunded         0                                      
Annual change in accrued RBA balance as of December 31, 2014 (and associated revenue taxes) to be collected         (9,200,000)                                      
Net annual incremental amount to be collected under the tariffs         (1,100,000)                                      
Impact on typical residential customer monthly bill         $ (90,000)                                      
Monthly utility usage assumption (in kilowatts per hour) | kWh         500                                      
Hawaii Electric Light Company, Inc. (HELCO)                                                
Public Utilities, Phase-in Plans [Abstract]                                                
Annual incremental RAM adjusted revenues         $ 1,500,000                                      
Annual change in accrued earnings sharing credits to be refunded         0                                      
Annual change in accrued RBA balance as of December 31, 2014 (and associated revenue taxes) to be collected         100,000                                      
Net annual incremental amount to be collected under the tariffs         1,500,000                                      
Impact on typical residential customer monthly bill         $ 880,000                                      
Monthly utility usage assumption (in kilowatts per hour) | kWh         500                                      
Maui Electric                                                
Environmental regulation                                                
Additional accrued investigation and estimated cleanup costs                   $ 3,600,000               $ 3,600,000 $ 3,600,000          
Public Utilities, Phase-in Plans [Abstract]                                                
Annual incremental RAM adjusted revenues         $ 1,500,000                                      
Annual change in accrued earnings sharing credits to be refunded         (100,000)                                      
Annual change in accrued RBA balance as of December 31, 2014 (and associated revenue taxes) to be collected         (2,200,000)                                      
Net annual incremental amount to be collected under the tariffs         (800,000)                                      
Impact on typical residential customer monthly bill         (130,000)                                      
Maui Electric | Subsequent Event                                                
Changes in the asset retirement obligation liability                                                
Revenue requirement associated with 2015 Plant additions   $ 4,300,000                                            
Lanai and Molokai                                                
Public Utilities, Phase-in Plans [Abstract]                                                
Impact on typical residential customer monthly bill         $ 0.11                                      
Monthly utility usage assumption (in kilowatts per hour) | kWh         400