Subsequent events (Details) $ in Millions	1 Months Ended		3 Months Ended		12 Months Ended
	Jan. 31, 2021 USD ($) retiree	Dec. 31, 2020 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Subsequent Event [Line Items]
One-time, non-cash, pre-tax settlement charge				$ 142.1
Pension Plans | United States
Subsequent Event [Line Items]
One-time, non-cash, pre-tax settlement charge		$ 142.1			$ 142.1	$ 0.0	$ 0.0
Subsequent Event | Pension Plans | United States
Subsequent Event [Line Items]
Agreement with insurance company, group annuity contract purchase amount	$ 279.0
Agreement with insurance company, number of retirees | retiree	11,600
Subsequent Event | Pension Plans | United States | Forecast
Subsequent Event [Line Items]
One-time, non-cash, pre-tax settlement charge			$ 158.0