Retirement benefits - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Vested percentage of interest of each affected participant after participating employer terminates participation	100.00%
Number of years for which regulatory asset/liability for each utility will be amortized, beginning with respective utility's next rate case	5 years
Executive life and nonqualified pension plan expenses	$ 1,000,000	$ 1,100,000
Regulatory asset charges pretax	$ 11,200,000	(128,000,000)
Fair value of plan assets, valuation difference amortized in first year	0.00%
Fair value of plan assets, valuation difference amortized in two to five years	25.00%
Number of past years for adding or subtracting the unamortized differences from fair value	4 years
Percentage of range around fair value	15.00%
Expected cash funding for qualified defined benefit plans
Defined contribution plan, expenses recognized	$ 7,000,000	7,000,000	$ 5,000,000
Cash contributions by the employer to defined contribution plan	7,000,000	6,000,000	5,000,000
Hawaiian Electric Company, Inc. and Subsidiaries
Expected cash funding for qualified defined benefit plans
Defined contribution plan, expenses recognized	$ 2,300,000	2,000,000	1,500,000
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Expected rate of return on plan assets	7.25%
Discount rate	4.51%
Assumed discount rate, active manager premium	0.20%
Pension expense	$ 59,000,000	33,000,000	33,000,000
Defined benefit pension plans with accumulated benefit obligations in excess of plan assets
Defined benefit plans - ABOs	1,700,000,000	1,800,000,000
Aggregate accumulated benefit obligations	1,600,000,000	1,700,000,000
Plan assets	1,400,000,000	1,500,000,000
Defined benefit plans with benefit obligations in excess of plan assets
PBOs	1,900,000,000	2,000,000,000
Plan assets	1,400,000,000	1,500,000,000
Expected cash funding for qualified defined benefit plans
Estimate of contributions to postretirement benefit plans in 2018	$ 47,000,000
Pension benefits | Hawaiian Electric Company, Inc. and Subsidiaries
Defined Benefit Plan Disclosure [Line Items]
Actual net return on plan assets	6.50%
Pension expense	$ 55,000,000	30,000,000	30,000,000
Defined benefit pension plans with accumulated benefit obligations in excess of plan assets
Defined benefit plans - ABOs	1,600,000,000	1,700,000,000
Aggregate accumulated benefit obligations	1,600,000,000	1,700,000,000
Plan assets	1,300,000,000	1,500,000,000
Defined benefit plans with benefit obligations in excess of plan assets
PBOs	1,800,000,000	1,900,000,000
Plan assets	1,300,000,000	$ 1,500,000,000
Expected cash funding for qualified defined benefit plans
Estimate of contributions to postretirement benefit plans in 2018	$ 47,000,000
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Assumed health care trend rate for medical in next fiscal year	7.25%	7.50%
Assumed health care trend rate for grading down plan in 12 fiscal years thereafter	5.00%
Assumed health care trend rate for dental in next fiscal year	5.00%	5.00%
Assumed health care trend rate for vision in next fiscal year	4.00%	4.00%
Assumed health care trend rate for grading down in 2028 and thereafter		5.00%
Postretirement benefits other than pension expense	$ 0	$ 1,000,000	1,000,000
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased total service and interest cost	100,000
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased the accumulated postretirement benefit obligation (APBO)	2,900,000
Effect of one-percentage-point decrease that would have reduced total service and interest cost	100,000
Effect of one-percentage-point decrease that would have reduced APBO	3,300,000
Expected cash funding for qualified defined benefit plans
Estimate of contributions to postretirement benefit plans in 2018	0
2019	88,000,000
2020	91,000,000
2021	95,000,000
2022	99,000,000
2023	103,000,000
2024 through 2028	574,000,000
Other benefits | Hawaiian Electric Company, Inc. and Subsidiaries
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits other than pension expense	(100,000)	$ 800,000	$ 700,000
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased total service and interest cost	100,000
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased the accumulated postretirement benefit obligation (APBO)	2,800,000
Effect of one-percentage-point decrease that would have reduced total service and interest cost	100,000
Effect of one-percentage-point decrease that would have reduced APBO	3,200,000
Expected cash funding for qualified defined benefit plans
Estimate of contributions to postretirement benefit plans in 2018	0
2019	81,000,000
2020	83,000,000
2021	87,000,000
2022	90,000,000
2023	93,000,000
2024 through 2028	$ 525,000,000
Minimum
Defined Benefit Plan Disclosure [Line Items]
Threshold percentage around fair value	85.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Threshold percentage around fair value	115.00%