Revenue from contracts with customers Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 907.7	$ 852.4	$ 849.7	$ 879.1	$ 903.2	$ 849.5	$ 805.9	$ 788.4	$ 3,488.9	$ 3,347.0	$ 3,020.6
Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									51.4	154.1	109.4
Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,720.6
High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,581.3
Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									187.0
Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									3,437.5	3,192.9	2,908.4
Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,702.2
Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,548.3
Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									187.0
Other items not allocated to segments | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									51.4	154.1	109.4
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 | 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									1,466.3	1,254.2	1,210.3
North America | Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									895.1
North America | Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									571.2
North America | Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									0.0
South America | Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									926.9	924.6	718.7
South America | Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									449.8
South America | Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									465.1
South America | Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									12.0
Rest of World | Reportable Segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									1,044.3	$ 1,014.1	$ 979.4
Rest of World | Reportable Segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									357.3
Rest of World | Reportable Segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									512.0
Rest of World | Reportable Segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service									$ 175.0