Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 905.0	$ 879.1
Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	484.4	419.7
High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	384.7	402.1
Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	35.9	57.3
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	905.0	853.3
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	484.4	409.0
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	384.7	387.0
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	35.9	57.3
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	434.5	320.1
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	277.2	190.0
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	157.3	130.1
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
Reportable segments | South America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	230.3	254.8
Reportable segments | South America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	119.2	125.4
Reportable segments | South America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	108.1	126.5
Reportable segments | South America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	3.0	2.9
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	240.2	278.4
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	88.0	93.6
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	119.3	130.4
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	32.9	54.4
Other items not allocated to segments | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	0.0	25.8
Other items not allocated to segments | Core services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	0.0	10.7
Other items not allocated to segments | High-value services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	0.0	15.1
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	$ 0.0