Retirement benefits - Asset Allocation Target and Actual Percentage (Details)	Dec. 31, 2024	Dec. 31, 2023
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Target	100.00%	100.00%
Pension benefits | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	48.00%	56.00%
Target	46.00%	50.00%
Pension benefits | U.S. equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target	8.00%	40.00%
Broad range allocation	43.00%	60.00%
Pension benefits | U.S. equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	100.00%	65.00%
Broad range allocation	83.00%
Pension benefits | Non-U.S equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	19.00%	13.00%
Target	17.00%	15.00%
Pension benefits | Non-U.S equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target	0.00%	5.00%
Pension benefits | Non-U.S equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	37.00%	25.00%
Pension benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	30.00%	28.00%
Target	32.00%	30.00%
Pension benefits | Fixed income securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target	11.00%	20.00%
Pension benefits | Fixed income securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	51.00%	60.00%
Pension benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	3.00%	3.00%
Target	5.00%	5.00%
Pension benefits | Private equity | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target	0.00%	0.00%
Pension benefits | Private equity | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	10.00%	10.00%
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Target	100.00%	100.00%
Other benefits | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	54.00%	58.00%
Target	54.00%	50.00%
Other benefits | U.S. equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target	14.00%	40.00%
Broad range allocation	54.00%
Other benefits | U.S. equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	100.00%	65.00%
Broad range allocation	94.00%	80.00%
Other benefits | Non-U.S equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	21.00%	15.00%
Target	20.00%	15.00%
Other benefits | Non-U.S equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target	0.00%	5.00%
Other benefits | Non-U.S equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	40.00%	25.00%
Other benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	25.00%	27.00%
Target	26.00%	30.00%
Other benefits | Fixed income securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target	6.00%	20.00%
Other benefits | Fixed income securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	46.00%	60.00%
Other benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	0.00%	0.00%
Target	0.00%	5.00%
Other benefits | Private equity | Minimum
Defined Benefit Plan Disclosure [Line Items]
Target		0.00%
Other benefits | Private equity | Maximum
Defined Benefit Plan Disclosure [Line Items]
Target	0.00%	10.00%