Income taxes - Deferred tax asset, net of valuation allowance and Uncertain tax positions (Details) $ in Millions	Mar. 31, 2019 USD ($)
Income taxes
Net deferred tax liability, net of valuation allowance	$ 59.0
Other net deferred tax liabilities	4.5
Uncertain tax positions
Total reserve for unrecognized tax benefits	63.3
Reversal of reserves for unrecognized tax benefits on permanent differences and interest and penalties	63.2
Reversal of reserves for unrecognized tax benefits on temporary differences	0.1
U.S. subsidiaries
Income taxes
Net deferred tax assets	33.1
Luxembourg subsidiaries
Income taxes
Net deferred tax assets	131.5
United Kingdom subsidiaries
Income taxes
Net deferred tax assets	11.5
Sweden subsidiaries
Income taxes
Net deferred tax assets	230.6
Swedish Tax Authority | Sweden subsidiaries
Uncertain tax positions
Reserve attributable to uncertain tax positions	$ 63.0