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Retirement benefits - Changes in Projected Benefit Obligations and Fair Value of Plan Assets and Amounts Recognized in OCI (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Hawaiian Electric Company, Inc. and Subsidiaries          
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Defined benefit pension and other postretirement benefit plans liability       $ (472,948) $ (599,726)
Pension benefits          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, balance at the beginning of the period $ 1,935,494 $ 1,798,030      
Service cost 64,906 60,555 $ 66,260    
Interest cost 81,185 81,549 76,960    
Actuarial losses (gains) 87,399 67,741      
Participants contributions 0 0      
Benefits paid and expenses (74,628) (72,381)      
Benefit obligation, balance at the end of the period 2,094,356 1,935,494 1,798,030    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 1,369,701 1,271,474      
Actual return on plan assets 255,324 103,836      
Employer contributions 66,983 65,463      
Participants contributions 0 0      
Benefits paid and expenses (73,305) (71,072)      
Fair value of plan assets, balance at the end of the period 1,618,703 1,369,701 1,271,474    
Accrued benefit asset (liability), balance       (475,653) (565,793)
Other assets       15,443 13,477
Defined benefit pension and other postretirement benefit plans liability       (491,096) (579,270)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income [Roll Forward]          
AOCI debit/(credit), balance at beginning of the period (excluding impact of PUC D&Os) 619,451 581,763      
Recognized during year – prior service credit 55 57      
Recognized during year – net actuarial losses (26,496) (24,832)      
Occurring during year – net actuarial losses (gains) (65,180) 62,463      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 527,830 619,451 581,763    
Cumulative impact of PUC D&Os       (489,894) (576,933)
AOCI debit/(credit), balance at end of the period       37,936 42,518
Net actuarial loss       527,907 619,582
Prior service gain       (77) (131)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 619,451 581,763 581,763 527,830 619,451
Income taxes (benefits)       (9,986) (16,746)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       27,950 25,772
Pension benefits | Hawaiian Electric Company, Inc. and Subsidiaries          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, balance at the beginning of the period 1,779,626 1,649,690      
Service cost 63,059 58,796 64,262    
Interest cost 74,632 74,808 70,529    
Actuarial losses (gains) 80,186 63,121      
Participants contributions 0 0      
Benefits paid and expenses (68,691) (66,789)      
Transfers (164) 0      
Benefit obligation, balance at the end of the period 1,928,648 1,779,626 1,649,690    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 1,233,184 1,141,833      
Actual return on plan assets 237,830 93,441      
Employer contributions 65,669 64,236      
Participants contributions 0 0      
Benefits paid and expenses (68,225) (66,326)      
Other (55) 0      
Fair value of plan assets, balance at the end of the period 1,468,403 1,233,184 1,141,833    
Accrued benefit asset (liability), balance       (460,245) (546,442)
Other liabilities (short-term)       (494) (460)
Defined benefit pension and other postretirement benefit plans liability       (459,751) (545,982)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income [Roll Forward]          
AOCI debit/(credit), balance at beginning of the period (excluding impact of PUC D&Os) 579,725 541,118      
Recognized during year – prior service credit (8) (13)      
Recognized during year – net actuarial losses (24,392) (22,693)      
Occurring during year – net actuarial losses (gains) (61,861) 61,313      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 493,464 579,725 541,118    
Cumulative impact of PUC D&Os       (489,894) (576,933)
AOCI debit/(credit), balance at end of the period       3,570 2,792
Net actuarial loss       493,439 579,691
Prior service gain       25 34
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 579,725 541,118 541,118 493,464 579,725
Income taxes (benefits)       (920) (1,087)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       2,650 1,705
Other benefits          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, balance at the beginning of the period 233,835 221,540      
Service cost 3,374 3,331 3,927    
Interest cost 9,453 9,670 9,011    
Actuarial losses (gains) (25,557) 7,831      
Participants contributions 2,078 1,405      
Benefits paid and expenses (10,582) (9,942)      
Benefit obligation, balance at the end of the period 212,601 233,835 221,540    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 174,251 170,687      
Actual return on plan assets 28,248 11,352      
Employer contributions 0 42      
Participants contributions 2,078 1,405      
Benefits paid and expenses (10,582) (9,235)      
Fair value of plan assets, balance at the end of the period 193,995 174,251 170,687    
Accrued benefit asset (liability), balance       (18,606) (59,584)
Other assets       0 0
Defined benefit pension and other postretirement benefit plans liability       (18,606) (59,584)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income [Roll Forward]          
AOCI debit/(credit), balance at beginning of the period (excluding impact of PUC D&Os) 42,290 32,550      
Recognized during year – prior service credit 1,793 1,793      
Recognized during year – net actuarial losses (1,130) (804)      
Occurring during year – net actuarial losses (gains) (41,479) 8,751      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 1,474 42,290 32,550    
Cumulative impact of PUC D&Os       (2,767) (43,974)
AOCI debit/(credit), balance at end of the period       (1,293) (1,684)
Net actuarial loss       10,183 52,792
Prior service gain       (8,709) (10,502)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 42,290 32,550 32,550 1,474 42,290
Income taxes (benefits)       333 656
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       (960) (1,028)
Other benefits | Hawaiian Electric Company, Inc. and Subsidiaries          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, balance at the beginning of the period 225,723 213,990      
Service cost 3,353 3,284 3,870    
Interest cost 9,115 9,337 8,700    
Actuarial losses (gains) (25,172) 7,545      
Participants contributions 2,047 1,389      
Benefits paid and expenses (10,419) (9,822)      
Transfers (3) 0      
Benefit obligation, balance at the end of the period 204,644 225,723 213,990    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 171,383 167,930      
Actual return on plan assets 27,806 11,168      
Employer contributions 0 11      
Participants contributions 2,047 1,389      
Benefits paid and expenses (10,419) (9,115)      
Other (3) 0      
Fair value of plan assets, balance at the end of the period 190,814 171,383 167,930    
Accrued benefit asset (liability), balance       (13,830) (54,340)
Other liabilities (short-term)       (633) (596)
Defined benefit pension and other postretirement benefit plans liability       (13,197) (53,744)
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income [Roll Forward]          
AOCI debit/(credit), balance at beginning of the period (excluding impact of PUC D&Os) 40,967 31,485      
Recognized during year – prior service credit 1,804 1,803      
Recognized during year – net actuarial losses (1,102) (793)      
Occurring during year – net actuarial losses (gains) (40,830) 8,472      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 839 40,967 31,485    
Cumulative impact of PUC D&Os       (2,767) (43,974)
AOCI debit/(credit), balance at end of the period       (1,928) (3,007)
Net actuarial loss       9,531 51,463
Prior service gain       (8,692) (10,496)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) $ 40,967 $ 31,485 $ 31,485 839 40,967
Income taxes (benefits)       497 1,170
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       $ (1,431) $ (1,837)