Income taxes - Deferred tax asset, net of valuation allowance and Uncertain tax positions (Details) $ in Millions	Sep. 30, 2020 USD ($)
Income taxes
Net deferred tax liability, net of valuation allowance	$ 55.1
Other net deferred tax liabilities	7.6
Uncertain tax positions
Total reserve for unrecognized tax benefits	47.5
Reversal of reserves for unrecognized tax benefits on permanent differences and interest and penalties	47.3
Reversal of reserves for unrecognized tax benefits on temporary differences	0.2
U.S. subsidiaries
Income taxes
Net deferred tax assets	25.0
Luxembourg subsidiaries
Income taxes
Net deferred tax assets	141.3
Sweden subsidiaries
Income taxes
Net deferred tax assets	213.8
Sweden subsidiaries | Swedish tax authority
Uncertain tax positions
Reserve attributable to uncertain tax positions	$ 46.6