Segment information - Revenue, Properties and Outsourcing Assets by Geographic Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 2,054.4	$ 2,026.3	$ 2,222.8
Properties, net	86.5	110.5	116.0
Outsourcing assets, net	124.6	173.9	202.1
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	856.2	781.5	824.0
Properties, net	62.5	82.0	82.3
Outsourcing assets, net	66.2	93.1	99.5
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	284.9	228.0	334.3
Outsourcing assets, net	36.3	55.3	71.7
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
Outsourcing assets, net	16.7	19.3	21.5
Other foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	913.3	1,016.8	1,064.5
Properties, net	24.0	28.5	33.7
Outsourcing assets, net	$ 5.4	$ 6.2	$ 9.4