Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 1,136.7	$ 1,075.5	$ 3,344.6	$ 3,102.0
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	1,136.7	1,075.5	3,344.6	3,102.0
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	523.2	525.1	1,568.9	1,542.7
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	561.2	495.3	1,618.6	1,402.9
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	52.3	55.1	157.1	156.4
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	400.6	360.7	1,171.0	1,034.6
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	198.7	178.1	579.9	534.0
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	201.9	182.6	591.1	500.6
Reportable segments | North America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	0.0	0.0	0.0	0.0
Reportable segments | Latin America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	301.1	289.3	898.7	831.8
Reportable segments | Latin America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	175.8	171.0	528.3	495.2
Reportable segments | Latin America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	119.2	113.3	352.6	323.3
Reportable segments | Latin America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	6.1	5.0	17.8	13.3
Reportable segments | Europe
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	220.0	238.0	668.8	683.2
Reportable segments | Europe | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	94.2	119.4	298.1	346.8
Reportable segments | Europe | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	92.0	81.8	269.2	230.9
Reportable segments | Europe | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	33.8	36.8	101.5	105.5
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	215.0	187.5	606.1	552.4
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	54.5	56.6	162.6	166.7
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	148.1	117.6	405.7	348.1
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 12.4	$ 13.3	$ 37.8	$ 37.6