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Retirement benefits - ABO and PBO in Excess of Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Defined benefit plans with PBOs in excess of plan assets |  |  |
Defined contribution plan, expenses recognized | $ 6 | $ 7 | $ 7
Pension benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plans - ABOs | 2,300 | 2,300 |
Defined benefit plans with ABO in excess of plan assets |  |  |
ABOs | 0 | 2,100 |
Fair value of plan assets | 0 | 2,000 |
Defined benefit plans with PBOs in excess of plan assets |  |  |
PBOs | 2,500 | 2,500 |
Fair value of plan assets | 2,200 | 2,000 |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |
Defined benefit plans with PBOs in excess of plan assets |  |  |
Defined contribution plan, expenses recognized | 3 | 3 | $ 3
Hawaiian Electric Company, Inc. and Subsidiaries | Pension benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined benefit plans - ABOs | 2,100 | 2,100 |
Defined benefit plans with ABO in excess of plan assets |  |  |
ABOs | 0 | 2,100 |
Fair value of plan assets | 0 | 1,900 |
Defined benefit plans with PBOs in excess of plan assets |  |  |
PBOs | 2,500 | 2,400 |
Fair value of plan assets | $ 2,100 | $ 1,900 |

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