Segment information - Revenue, Properties and Outsourcing Assets by Geographic Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 2,015.4	$ 1,979.9	$ 2,054.4
Properties, net	64.3	75.9	86.5
Outsourcing assets, net	31.6	66.4	124.6
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	889.0	854.9	856.2
Properties, net	45.9	52.5	62.5
Outsourcing assets, net	19.5	36.0	66.2
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	289.3	228.0	284.9
Outsourcing assets, net	2.0	17.9	36.3
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
Outsourcing assets, net	7.8	9.5	16.7
Other foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	837.1	897.0	913.3
Properties, net	18.4	23.4	24.0
Outsourcing assets, net	$ 2.3	$ 3.0	$ 5.4