Retirement benefits - Weighted Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pension benefits
Benefit obligation
Discount rate	5.77%	5.35%	5.67%
Rate of compensation increase	3.50%	3.50%	3.50%
Net periodic pension/benefit cost (years ended)
Discount rate	5.35%	5.67%	3.05%
Expected return on plan assets (gross return)	7.25%	7.25%	7.25%
Rate of compensation increase	3.50%	3.50%	3.50%
Other benefits
Benefit obligation
Discount rate	5.72%	5.39%	5.66%
Net periodic pension/benefit cost (years ended)
Discount rate	5.39%	5.66%	3.07%
Expected return on plan assets (gross return)	7.25%	7.25%	7.25%