Retirement benefits - ABO and PBO in Excess of Plan Assets (Details) - Pension benefits - USD ($) $ in Billions	Dec. 31, 2024	Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans - ABOs	$ 1.7	$ 1.8
Defined benefit pension plans with PBOs in excess of plan assets2
PBOs	1.9	2.0
Fair value of plan assets	1.9	1.9
Hawaiian Electric Company, Inc. and Subsidiaries
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans - ABOs	1.7	1.7
Defined benefit pension plans with PBOs in excess of plan assets2
PBOs	0.0	1.9
Fair value of plan assets	$ 0.0	$ 1.8

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

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Amount of plan asset for defined benefit pension plan with projected benefit obligation in excess of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480482/715-20-55-17

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-3

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Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets.

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