Retirement benefits - Weighted Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Benefit obligation
Rate of compensation increase	3.50%	3.50%	3.50%
Net periodic pension/benefit cost (years ended)
Rate of compensation increase	3.50%	3.50%	3.50%
Pension benefits
Benefit obligation
Discount rate	3.61%	4.31%	3.74%
Net periodic pension/benefit cost (years ended)
Discount rate	4.31%	3.74%	4.26%
Expected return on plan assets	7.25%	7.50%	7.50%
Pension benefits | American Savings Bank (ASB)
Net periodic pension/benefit cost (years ended)
Expected return on plan assets	4.51%	3.94%	4.46%
Other benefits
Benefit obligation
Discount rate	3.52%	4.34%	3.72%
Net periodic pension/benefit cost (years ended)
Discount rate	4.34%	3.72%	4.22%
Expected return on plan assets	7.25%	7.50%	7.50%