Segment information - Revenue, Properties and Outsourcing Assets by Geographic Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 576.9	$ 495.2	$ 438.8	$ 515.4	$ 546.8	$ 552.1	$ 569.4	$ 554.5	$ 2,026.3	$ 2,222.8	$ 2,251.2
Properties, net	110.5				116.0				110.5	116.0	111.9
Outsourcing assets, net	173.9				202.1				173.9	202.1	216.4
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									781.5	824.0	666.2
Properties, net	82.0				82.3				82.0	82.3	75.9
Outsourcing assets, net	93.1				99.5				93.1	99.5	97.6
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									228.0	334.3	360.7
Properties, net	9.2				10.5				9.2	10.5	5.3
Outsourcing assets, net	55.3				71.7				55.3	71.7	86.5
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
Outsourcing assets, net	19.3				21.5				19.3	21.5	21.7
Other foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,016.8	1,064.5	1,224.3
Properties, net	19.3				23.2				19.3	23.2	30.7
Outsourcing assets, net	$ 6.2				$ 9.4				$ 6.2	$ 9.4	$ 10.6