Retirement benefits - Asset Allocation Target and Actual Percentage (Details)	Dec. 31, 2021	Dec. 31, 2020
U.S. equity securities | Minimum
Asset category, Target
Target	60.00%
U.S. equity securities | Maximum
Asset category, Target
Target	80.00%
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Asset category, Target
Target	100.00%
Pension benefits | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	59.00%	58.00%
Asset category, Target
Target	52.00%
Pension benefits | U.S. equity securities | Minimum
Asset category, Target
Target	45.00%
Pension benefits | U.S. equity securities | Maximum
Asset category, Target
Target	65.00%
Pension benefits | Non-U.S equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	13.00%	14.00%
Asset category, Target
Target	15.00%
Pension benefits | Non-U.S equity securities | Minimum
Asset category, Target
Target	5.00%
Pension benefits | Non-U.S equity securities | Maximum
Asset category, Target
Target	25.00%
Pension benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	27.00%	28.00%
Asset category, Target
Target	30.00%
Pension benefits | Fixed income securities | Minimum
Asset category, Target
Target	20.00%
Pension benefits | Fixed income securities | Maximum
Asset category, Target
Target	40.00%
Pension benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	1.00%	0.00%
Asset category, Target
Target	3.00%
Pension benefits | Private equity | Minimum
Asset category, Target
Target	0.00%
Pension benefits | Private equity | Maximum
Asset category, Target
Target	5.00%
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Asset category, Target
Target	100.00%
Other benefits | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	58.00%	57.00%
Asset category, Target
Target	52.00%
Other benefits | U.S. equity securities | Minimum
Asset category, Target
Target	45.00%
Other benefits | U.S. equity securities | Maximum
Asset category, Target
Target	65.00%
Other benefits | Non-U.S equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	15.00%	16.00%
Asset category, Target
Target	15.00%
Other benefits | Non-U.S equity securities | Minimum
Asset category, Target
Target	5.00%
Other benefits | Non-U.S equity securities | Maximum
Asset category, Target
Target	25.00%
Other benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	27.00%	27.00%
Asset category, Target
Target	30.00%
Other benefits | Fixed income securities | Minimum
Asset category, Target
Target	20.00%
Other benefits | Fixed income securities | Maximum
Asset category, Target
Target	40.00%
Other benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	0.00%	0.00%
Asset category, Target
Target	3.00%
Other benefits | Private equity | Minimum
Asset category, Target
Target	0.00%
Other benefits | Private equity | Maximum
Asset category, Target
Target	5.00%