Retirement benefits - Weighted Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Benefit obligation
Rate of compensation increase	3.50%	3.50%	3.50%
Pension benefits
Benefit obligation
Discount rate	2.92%	3.61%	4.31%
Net periodic pension/benefit cost (years ended)
Discount rate	3.61%	4.31%	3.74%
Expected return on plan assets	7.25%	7.25%	7.50%
Rate of compensation increase	3.50%	3.50%	3.50%
Pension benefits | American Savings Bank (ASB)
Benefit obligation
Discount rate	2.76%	3.49%
Net periodic pension/benefit cost (years ended)
Discount rate	3.49%
Expected return on plan assets	3.69%	4.51%	3.94%
Other benefits
Benefit obligation
Discount rate	2.83%	3.52%	4.34%
Net periodic pension/benefit cost (years ended)
Discount rate	3.52%	4.34%	3.72%
Expected return on plan assets	7.25%	7.25%	7.50%