Income taxes - Income before income tax by jurisdiction (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Domestic							$ 178.5	$ 113.6	$ 198.0
Foreign							(133.5)	38.3	3.3
Income before income tax (expense) benefit	$ (153.9)	$ (60.4)	$ 79.7	$ 179.7	$ 259.4	$ 199.0	45.1	151.9	201.3
U.S.
Income Taxes [Line Items]
Foreign							21.8	38.1	3.2
U.K.
Income Taxes [Line Items]
Foreign							1.9	0.2	0.1
Sweden
Income Taxes [Line Items]
Foreign							(138.1)	0.0	0.0
Luxembourg
Income Taxes [Line Items]
Foreign							(20.5)	0.0	0.0
Other
Income Taxes [Line Items]
Foreign							$ 1.5	$ 0.0	$ 0.0