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Retirement benefits - Weighted Average Assumptions (Details)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension benefits      
Benefit obligation      
Discount rate 5.35% 5.67% 3.05%
Rate of compensation increase 3.50% 3.50% 3.50%
Net periodic pension/benefit cost (years ended)      
Discount rate 5.67% 3.05% 2.92%
Expected return on plan assets 7.25% 7.25% 7.25%
Rate of compensation increase 3.50% 3.50% 3.50%
Pension benefits | American Savings Bank (ASB)      
Benefit obligation      
Discount rate 5.39% 5.63% 3.04%
Net periodic pension/benefit cost (years ended)      
Discount rate 5.63% 3.04% 2.76%
Expected return on plan assets 5.83% 3.24% 2.96%
Other benefits      
Benefit obligation      
Discount rate 5.39% 5.66% 3.07%
Net periodic pension/benefit cost (years ended)      
Discount rate 5.66% 3.07% 2.83%
Expected return on plan assets 7.25% 7.25% 7.25%