Retirement benefits - Weighted Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension benefits
Benefit obligation
Discount rate	3.05%	2.92%	3.61%
Rate of compensation increase	3.50%	3.50%	3.50%
Net periodic pension/benefit cost (years ended)
Discount rate	2.92%	3.61%	4.31%
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	3.04%	2.76%	3.49%
Net periodic pension/benefit cost (years ended)
Discount rate	2.76%	3.49%
Expected return on plan assets	2.96%	3.69%	4.51%
Other benefits
Benefit obligation
Discount rate	3.07%	2.83%	3.52%
Net periodic pension/benefit cost (years ended)
Discount rate	2.83%	3.52%	4.34%
Expected return on plan assets	7.25%	7.25%	7.25%