Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 879.1	$ 788.4
Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	419.7	371.9
High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	402.1	368.7
Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	57.3	47.8
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	853.3	740.3
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	409.0	347.8
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	387.0	344.7
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	57.3	47.8
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	320.1	304.6
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	190.0	172.3
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	130.1	132.3
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	254.8	202.2
Reportable segments | South America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	125.4	96.8
Reportable segments | South America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	126.5	101.4
Reportable segments | South America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	2.9	4.0
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	278.4	233.5
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	93.6	78.7
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	130.4	111.0
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	54.4	43.8
Other items not allocated to segments | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	25.8	48.1
Other items not allocated to segments | Core services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	10.7	24.1
Other items not allocated to segments | High-value services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	15.1	24.0
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	$ 0.0