Revenue from contracts with customers - Disaggregation of revenue (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
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 1,021.6	$ 970.5	$ 826.0	$ 872.8	$ 935.8	$ 928.4	$ 914.0	$ 905.0	$ 3,690.9	$ 3,683.2	$ 3,488.9
Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service										1,953.3	1,720.6
High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service										1,584.4	1,581.3
Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service										145.5	187.0
Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									3,690.9	3,679.7	3,437.5
Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,909.1	1,953.3	1,702.2
Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,613.7	1,580.9	1,548.3
Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									168.1	145.5	187.0
Other items not allocated to segments
Disaggregation of Revenue [Line Items]
Acquisitions and dispositions, revenues									0.0	(0.5)	0.0
Other items not allocated to segments | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									0.0	0.0	51.4
Other items not allocated to segments | Core services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service											18.4
Other items not allocated to segments | High-value services
Disaggregation of Revenue [Line Items]
Acquisitions and dispositions, revenues										(0.5)
Other items not allocated to segments | High-value services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service											33.0
Other items not allocated to segments | Other security services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service											0.0
North America | Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,261.4	1,370.4	1,101.0
North America | Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									702.8	794.6	602.6
North America | Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									558.6	575.8	498.4
North America | Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									0.0	0.0	0.0
Latin America | Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,071.9	1,319.8	1,281.6
Latin America | Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									650.5	806.9	742.4
Latin America | Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									404.6	501.6	527.3
Latin America | Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									16.8	11.3	11.9
Europe | Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									753.8	549.6	612.6
Europe | Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									382.0	232.1	247.2
Europe | Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									239.0	185.5	192.4
Europe | Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									132.8	132.0	173.0
Rest of World | Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									603.8	439.9	442.3
Rest of World | Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									173.8	119.7	110.0
Rest of World | Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									411.5	318.0	330.2
Rest of World | Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									18.5	2.2	$ 2.1
Internal Loss AR Rebuild
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service						$ 4.0
Internal Loss AR Rebuild | Other items not allocated to segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									$ 0.0	4.0
Internal Loss AR Rebuild | Other items not allocated to segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service										$ 4.0