Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 1,075.5	$ 970.5	$ 3,102.0	$ 2,669.3
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	1,075.5	970.5	3,102.0	2,669.3
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	525.1	500.0	1,542.7	1,376.7
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	495.3	422.6	1,402.9	1,169.8
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	55.1	47.9	156.4	122.8
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	360.7	316.8	1,034.6	932.0
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	178.1	176.3	534.0	517.8
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	182.6	140.5	500.6	414.2
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	289.3	256.7	831.8	786.1
Reportable segments | Latin America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	171.0	154.9	495.2	479.0
Reportable segments | Latin America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	113.3	97.2	323.3	294.7
Reportable segments | Latin America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	5.0	4.6	13.3	12.4
Reportable segments | Europe
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	238.0	224.0	683.2	518.2
Reportable segments | Europe | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	119.4	115.4	346.8	259.7
Reportable segments | Europe | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	81.8	71.7	230.9	159.5
Reportable segments | Europe | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	36.8	36.9	105.5	99.0
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	187.5	173.0	552.4	433.0
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	56.6	53.4	166.7	120.2
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	117.6	113.2	348.1	301.4
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 13.3	$ 6.4	$ 37.6	$ 11.4