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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2020
Dec. 31, 2019
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       $ (530,532) $ (478,763)
Pension benefits          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, balance at the beginning of the period $ 2,278,283 $ 1,991,384      
Service cost 73,387 62,135 $ 68,987    
Interest cost 81,335 84,267 77,374    
Actuarial losses 275,973 224,421      
Participants contributions 0 0      
Benefits paid and expenses (84,448) (83,924)      
Benefit obligation, balance at the end of the period 2,624,530 2,278,283 1,991,384    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 1,799,200 1,479,067      
Actual return on plan assets 302,566 354,072      
Employer contributions 70,844 48,629      
Participants contributions 0 0      
Benefits paid and expenses (83,119) (82,568)      
Fair value of plan assets, balance at the end of the period 2,089,491 1,799,200 1,479,067    
Accrued benefit asset (liability), balance       (535,039) (479,083)
Other assets       25,851 19,396
Defined benefit pension and other postretirement benefit plans liability       (560,890) (498,479)
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) 503,821 536,920      
Recognized during year – prior service credit (cost) (8) 42      
Recognized during year – net actuarial (losses) gains (33,456) (15,479)      
Occurring during year – net actuarial losses (gains) 87,207 (17,662)      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 557,564 503,821 536,920    
Cumulative impact of PUC D&Os       (534,594) (474,628)
AOCI debit/(credit), balance at end of the period       22,970 29,193
Net actuarial loss       557,564 503,813
Prior service cost (gain)       0 8
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 557,564 536,920 536,920 557,564 503,821
Income taxes (benefits)       (5,988) (7,677)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       16,982 21,516
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 2,110,904 1,837,653      
Service cost 71,604 60,461 67,359    
Interest cost 75,484 77,851 71,294    
Actuarial losses 260,102 212,310      
Participants contributions 0 0      
Benefits paid and expenses (77,336) (77,060)      
Transfers 0 (311)      
Benefit obligation, balance at the end of the period 2,440,758 2,110,904 1,837,653    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 1,640,417 1,343,113      
Actual return on plan assets 276,453 326,204      
Employer contributions 69,720 47,808      
Participants contributions 0 0      
Benefits paid and expenses (76,860) (76,581)      
Other 0 (127)      
Fair value of plan assets, balance at the end of the period 1,909,730 1,640,417 1,343,113    
Accrued benefit asset (liability), balance       (531,028) (470,487)
Other liabilities (short-term)       (535) (518)
Defined benefit pension and other postretirement benefit plans liability       (530,493) (469,969)
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) 478,078 502,189      
Recognized during year – prior service credit (cost) (9) (7)      
Recognized during year – net actuarial (losses) gains (30,566) (14,658)      
Occurring during year – net actuarial losses (gains) 91,018 (9,446)      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 538,521 478,078 502,189    
Cumulative impact of PUC D&Os       (534,594) (474,628)
AOCI debit/(credit), balance at end of the period       3,927 3,450
Net actuarial loss       538,521 478,069
Prior service cost (gain)       0 9
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 478,078 478,078 502,189 538,521 478,078
Income taxes (benefits)       (1,011) (888)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       2,916 2,562
Other benefits          
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]          
Benefit obligation, balance at the beginning of the period 215,639 188,666      
Service cost 2,537 2,209 2,721    
Interest cost 7,407 8,004 7,933    
Actuarial losses 9,785 25,998      
Participants contributions 2,768 2,351      
Benefits paid and expenses (11,715) (11,589)      
Benefit obligation, balance at the end of the period 226,421 215,639 188,666    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 200,831 173,693      
Actual return on plan assets 27,678 35,525      
Employer contributions 0 0      
Participants contributions 2,768 2,351      
Benefits paid and expenses (11,404) (10,738)      
Fair value of plan assets, balance at the end of the period 219,873 200,831 173,693    
Accrued benefit asset (liability), balance       (6,548) (14,808)
Other assets       0 0
Defined benefit pension and other postretirement benefit plans liability       (6,548) (14,808)
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) 6,610 1,962      
Recognized during year – prior service credit (cost) 1,761 1,806      
Recognized during year – net actuarial (losses) gains (208) 13      
Occurring during year – net actuarial losses (gains) (5,768) 2,829      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 2,395 6,610 1,962    
Cumulative impact of PUC D&Os       (1,177) (7,458)
AOCI debit/(credit), balance at end of the period       1,218 (848)
Net actuarial loss       5,731 11,707
Prior service cost (gain)       (3,336) (5,097)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 6,610 6,610 1,962 2,395 6,610
Income taxes (benefits)       (313) 219
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       905 (629)
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 207,073 181,162      
Service cost 2,515 2,191 2,704    
Interest cost 7,103 7,673 7,628    
Actuarial losses 9,151 25,123      
Participants contributions 2,717 2,311      
Benefits paid and expenses (11,485) (11,382)      
Transfers 0 (5)      
Benefit obligation, balance at the end of the period 217,074 207,073 181,162    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets, balance at the beginning of the period 197,564 170,862      
Actual return on plan assets 27,207 34,928      
Employer contributions 0 0      
Participants contributions 2,717 2,311      
Benefits paid and expenses (11,173) (10,532)      
Other 0 (5)      
Fair value of plan assets, balance at the end of the period 216,315 197,564 170,862    
Accrued benefit asset (liability), balance       (759) (9,509)
Other liabilities (short-term)       (720) (715)
Defined benefit pension and other postretirement benefit plans liability       (39) (8,794)
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) 5,730 1,551      
Recognized during year – prior service credit (cost) 1,758 1,803      
Recognized during year – net actuarial (losses) gains (207) 0      
Occurring during year – net actuarial losses (gains) (6,100) 2,376      
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) 1,181 5,730 1,551    
Cumulative impact of PUC D&Os       (1,177) (7,458)
AOCI debit/(credit), balance at end of the period       4 (1,728)
Net actuarial loss       4,508 10,815
Prior service cost (gain)       (3,327) (5,085)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os) $ 1,181 $ 1,551 $ 1,551 1,181 5,730
Income taxes (benefits)       (1) 445
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period       $ 3 $ (1,283)