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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
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				$ (478,763)	$ (503,659)
Pension benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, balance at the beginning of the period	$ 1,991,384	$ 2,094,356
Service cost	62,135	68,987	$ 64,906
Interest cost	84,267	77,374	81,185
Actuarial losses (gains)	224,421	(171,226)
Participants contributions	0	0
Benefits paid and expenses	(83,924)	(78,107)
Benefit obligation, balance at the end of the period	2,278,283	1,991,384	2,094,356
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	1,479,067	1,618,703
Actual return on plan assets	354,072	(101,406)
Employer contributions	48,629	38,496
Participants contributions	0	0
Benefits paid and expenses	(82,568)	(76,726)
Fair value of plan assets, balance at the end of the period	1,799,200	1,479,067	1,618,703
Accrued benefit asset (liability), balance				(479,083)	(512,317)
Other assets				19,396	10,930
Defined benefit pension and other postretirement benefit plans liability				(498,479)	(523,247)
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)	536,920	527,830
Recognized during year – prior service credit	42	42
Recognized during year – net actuarial (losses) gains	(15,479)	(30,084)
Occurring during year – net actuarial losses (gains)	(17,662)	39,132
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	503,821	536,920	527,830
Cumulative impact of PUC D&Os				(474,628)	(498,944)
AOCI debit/(credit), balance at end of the period				29,193	37,976
Net actuarial loss				503,813	536,954
Prior service cost (gain)				8	(34)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	536,920	536,920	527,830	503,821	536,920
Income taxes (benefits)				(7,677)	(10,023)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				21,516	27,953
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	1,837,653	1,928,648
Service cost	60,461	67,359	63,059
Interest cost	77,851	71,294	74,632
Actuarial losses (gains)	212,310	(158,258)
Participants contributions	0	0
Benefits paid and expenses	(77,060)	(71,535)
Transfers	(311)	145
Benefit obligation, balance at the end of the period	2,110,904	1,837,653	1,928,648
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	1,343,113	1,468,403
Actual return on plan assets	326,204	(91,836)
Employer contributions	47,808	37,550
Participants contributions	0	0
Benefits paid and expenses	(76,581)	(71,060)
Other	(127)	56
Fair value of plan assets, balance at the end of the period	1,640,417	1,343,113	1,468,403
Accrued benefit asset (liability), balance				(470,487)	(494,540)
Other liabilities (short-term)				(518)	(512)
Defined benefit pension and other postretirement benefit plans liability				(469,969)	(494,028)
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)	502,189	493,464
Recognized during year – prior service credit	(7)	(8)
Recognized during year – net actuarial (losses) gains	(14,658)	(27,302)
Occurring during year – net actuarial losses (gains)	(9,446)	36,035
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	478,078	502,189	493,464
Cumulative impact of PUC D&Os				(474,628)	(498,944)
AOCI debit/(credit), balance at end of the period				3,450	3,245
Net actuarial loss				478,069	502,173
Prior service cost (gain)				9	16
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	478,078	502,189	493,464	478,078	502,189
Income taxes (benefits)				(888)	(836)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				2,562	2,409
Other benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, balance at the beginning of the period	188,666	212,601
Service cost	2,209	2,721	3,374
Interest cost	8,004	7,933	9,453
Actuarial losses (gains)	25,998	(25,977)
Participants contributions	2,351	2,505
Benefits paid and expenses	(11,589)	(11,117)
Benefit obligation, balance at the end of the period	215,639	188,666	212,601
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	173,693	193,995
Actual return on plan assets	35,525	(11,846)
Employer contributions	0	0
Participants contributions	2,351	2,505
Benefits paid and expenses	(10,738)	(10,961)
Fair value of plan assets, balance at the end of the period	200,831	173,693	193,995
Accrued benefit asset (liability), balance				(14,808)	(14,973)
Other assets				0	0
Defined benefit pension and other postretirement benefit plans liability				(14,808)	(14,973)
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)	1,962	1,474
Recognized during year – prior service credit	1,806	1,805
Recognized during year – net actuarial (losses) gains	13	(95)
Occurring during year – net actuarial losses (gains)	2,829	(1,222)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	6,610	1,962	1,474
Cumulative impact of PUC D&Os				(7,458)	(4,929)
AOCI debit/(credit), balance at end of the period				(848)	(2,967)
Net actuarial loss				11,707	8,865
Prior service cost (gain)				(5,097)	(6,903)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	6,610	1,474	1,474	6,610	1,962
Income taxes (benefits)				219	765
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				(629)	(2,202)
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	181,162	204,644
Service cost	2,191	2,704	3,353
Interest cost	7,673	7,628	9,115
Actuarial losses (gains)	25,123	(25,330)
Participants contributions	2,311	2,472
Benefits paid and expenses	(11,382)	(10,958)
Transfers	(5)	2
Benefit obligation, balance at the end of the period	207,073	181,162	204,644
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	170,862	190,814
Actual return on plan assets	34,928	(11,625)
Employer contributions	0	0
Participants contributions	2,311	2,472
Benefits paid and expenses	(10,532)	(10,801)
Other	(5)	2
Fair value of plan assets, balance at the end of the period	197,564	170,862	190,814
Accrued benefit asset (liability), balance				(9,509)	(10,300)
Other liabilities (short-term)				(715)	(669)
Defined benefit pension and other postretirement benefit plans liability				(8,794)	(9,631)
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)	1,551	839
Recognized during year – prior service credit	1,803	1,803
Recognized during year – net actuarial (losses) gains	0	(98)
Occurring during year – net actuarial losses (gains)	2,376	(993)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	5,730	1,551	839
Cumulative impact of PUC D&Os				(7,458)	(4,929)
AOCI debit/(credit), balance at end of the period				(1,728)	(3,378)
Net actuarial loss				10,815	8,439
Prior service cost (gain)				(5,085)	(6,888)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	$ 5,730	$ 839	$ 839	5,730	1,551
Income taxes (benefits)				445	870
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				$ (1,283)	$ (2,508)