Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 1,048.8	$ 826.0	$ 2,026.5	$ 1,698.8
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	1,048.8	826.0	2,026.5	1,698.8
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	512.1	420.1	1,017.6	876.7
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	481.6	365.4	907.6	747.2
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	55.1	40.5	101.3	74.9
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	356.8	274.3	673.9	615.2
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	176.5	147.3	355.9	341.5
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	180.3	127.0	318.0	273.7
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	272.8	230.4	542.5	529.4
Reportable segments | Latin America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	164.0	143.7	324.2	324.1
Reportable segments | Latin America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	104.5	82.6	210.0	197.5
Reportable segments | Latin America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	4.3	4.1	8.3	7.8
Reportable segments | Europe
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	230.8	167.9	445.2	294.2
Reportable segments | Europe | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	116.1	90.0	227.4	144.3
Reportable segments | Europe | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	78.0	46.0	149.1	87.8
Reportable segments | Europe | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	36.7	31.9	68.7	62.1
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	188.4	153.4	364.9	260.0
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	55.5	39.1	110.1	66.8
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	118.8	109.8	230.5	188.2
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 14.1	$ 4.5	$ 24.3	$ 5.0