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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
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				$ (503,659)	$ (472,948)
Pension benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, balance at the beginning of the period	$ 2,094,356	$ 1,935,494
Service cost	68,987	64,906	$ 60,555
Interest cost	77,374	81,185	81,549
Actuarial losses (gains)	(171,226)	87,399
Participants contributions	0	0
Benefits paid and expenses	(78,107)	(74,628)
Benefit obligation, balance at the end of the period	1,991,384	2,094,356	1,935,494
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	1,618,703	1,369,701
Actual return on plan assets	(101,406)	255,324
Employer contributions	38,496	66,983
Participants contributions	0	0
Benefits paid and expenses	(76,726)	(73,305)
Fair value of plan assets, balance at the end of the period	1,479,067	1,618,703	1,369,701
Accrued benefit asset (liability), balance				(512,317)	(475,653)
Other assets				10,930	15,443
Defined benefit pension and other postretirement benefit plans liability				(523,247)	(491,096)
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)	527,830	619,451
Recognized during year – prior service credit	42	55
Recognized during year – net actuarial losses	(30,084)	(26,496)
Occurring during year – net actuarial losses (gains)	39,132	(65,180)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	536,920	527,830	619,451
Cumulative impact of PUC D&Os				(498,944)	(489,894)
AOCI debit/(credit), balance at end of the period				37,976	37,936
Net actuarial loss				536,954	527,907
Prior service gain				(34)	(77)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	527,830	619,451	619,451	536,920	527,830
Income taxes (benefits)				(10,023)	(9,986)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				27,953	27,950
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	1,928,648	1,779,626
Service cost	67,359	63,059	58,796
Interest cost	71,294	74,632	74,808
Actuarial losses (gains)	(158,258)	80,186
Participants contributions	0	0
Benefits paid and expenses	(71,535)	(68,691)
Transfers	145	(164)
Benefit obligation, balance at the end of the period	1,837,653	1,928,648	1,779,626
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	1,468,403	1,233,184
Actual return on plan assets	(91,836)	237,830
Employer contributions	37,550	65,669
Participants contributions	0	0
Benefits paid and expenses	(71,060)	(68,225)
Other	56	(55)
Fair value of plan assets, balance at the end of the period	1,343,113	1,468,403	1,233,184
Accrued benefit asset (liability), balance				(494,540)	(460,245)
Other liabilities (short-term)				(512)	(494)
Defined benefit pension and other postretirement benefit plans liability				(494,028)	(459,751)
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)	493,464	579,725
Recognized during year – prior service credit	(8)	(8)
Recognized during year – net actuarial losses	(27,302)	(24,392)
Occurring during year – net actuarial losses (gains)	36,035	(61,861)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	502,189	493,464	579,725
Cumulative impact of PUC D&Os				(498,944)	(489,894)
AOCI debit/(credit), balance at end of the period				3,245	3,570
Net actuarial loss				502,173	493,439
Prior service gain				16	25
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	493,464	579,725	579,725	502,189	493,464
Income taxes (benefits)				(836)	(920)
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				2,409	2,650
Other benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, balance at the beginning of the period	212,601	233,835
Service cost	2,721	3,374	3,331
Interest cost	7,933	9,453	9,670
Actuarial losses (gains)	(25,977)	(25,557)
Participants contributions	2,505	2,078
Benefits paid and expenses	(11,117)	(10,582)
Benefit obligation, balance at the end of the period	188,666	212,601	233,835
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	193,995	174,251
Actual return on plan assets	(11,846)	28,248
Employer contributions	0	0
Participants contributions	2,505	2,078
Benefits paid and expenses	(10,961)	(10,582)
Fair value of plan assets, balance at the end of the period	173,693	193,995	174,251
Accrued benefit asset (liability), balance				(14,973)	(18,606)
Other assets				0	0
Defined benefit pension and other postretirement benefit plans liability				(14,973)	(18,606)
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)	1,474	42,290
Recognized during year – prior service credit	1,805	1,793
Recognized during year – net actuarial losses	(95)	(1,130)
Occurring during year – net actuarial losses (gains)	(1,222)	(41,479)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	1,962	1,474	42,290
Cumulative impact of PUC D&Os				(4,929)	(2,767)
AOCI debit/(credit), balance at end of the period				(2,967)	(1,293)
Net actuarial loss				8,865	10,183
Prior service gain				(6,903)	(8,709)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	1,474	42,290	42,290	1,962	1,474
Income taxes (benefits)				765	333
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				(2,202)	(960)
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	204,644	225,723
Service cost	2,704	3,353	3,284
Interest cost	7,628	9,115	9,337
Actuarial losses (gains)	(25,330)	(25,172)
Participants contributions	2,472	2,047
Benefits paid and expenses	(10,958)	(10,419)
Transfers	2	(3)
Benefit obligation, balance at the end of the period	181,162	204,644	225,723
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, balance at the beginning of the period	190,814	171,383
Actual return on plan assets	(11,625)	27,806
Employer contributions	0	0
Participants contributions	2,472	2,047
Benefits paid and expenses	(10,801)	(10,419)
Other	2	(3)
Fair value of plan assets, balance at the end of the period	170,862	190,814	171,383
Accrued benefit asset (liability), balance				(10,300)	(13,830)
Other liabilities (short-term)				(669)	(633)
Defined benefit pension and other postretirement benefit plans liability				(9,631)	(13,197)
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)	839	40,967
Recognized during year – prior service credit	1,803	1,804
Recognized during year – net actuarial losses	(98)	(1,102)
Occurring during year – net actuarial losses (gains)	(993)	(40,830)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	1,551	839	40,967
Cumulative impact of PUC D&Os				(4,929)	(2,767)
AOCI debit/(credit), balance at end of the period				(3,378)	(1,928)
Net actuarial loss				8,439	9,531
Prior service gain				(6,888)	(8,692)
AOCI debit/(credit), balance at end of the period (excluding impact of PUC D&Os)	$ 839	$ 40,967	$ 40,967	1,551	839
Income taxes (benefits)				870	497
AOCI debit/(credit), net of taxes (benefits) balance at the end of the period				$ (2,508)	$ (1,431)