Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 849.7	$ 805.9	$ 1,728.8	$ 1,594.3
Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	399.7	381.3	819.4	753.2
High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	397.2	371.7	799.3	740.4
Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	52.8	52.9	110.1	100.7
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	824.1	759.6	1,677.4	1,499.9
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	392.0	357.0	801.0	704.8
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	379.3	349.7	766.3	694.4
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	52.8	52.9	110.1	100.7
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	324.0	311.0	644.1	615.6
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	189.8	182.9	379.8	355.2
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	134.2	128.1	264.3	260.4
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 | South America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	233.3	204.6	488.1	406.8
Reportable segments | South America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	113.9	97.0	239.3	193.8
Reportable segments | South America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	116.2	104.6	242.7	206.0
Reportable segments | South America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	3.2	3.0	6.1	7.0
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	266.8	244.0	545.2	477.5
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	88.3	77.1	181.9	155.8
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	128.9	117.0	259.3	228.0
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	49.6	49.9	104.0	93.7
Other items not allocated to segments | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	25.6	46.3	51.4	94.4
Other items not allocated to segments | Core services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	7.7	24.3	18.4	48.4
Other items not allocated to segments | High-value services | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	17.9	22.0	33.0	46.0
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	$ 0.0	$ 0.0