Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 852.4	$ 849.5	$ 2,581.2	$ 2,443.8
Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	430.1	401.1	1,249.5	1,154.3
High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	383.0	389.9	1,182.3	1,130.3
Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	39.3	58.5	149.4	159.2
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	852.4	828.7	2,529.8	2,328.6
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	430.1	389.5	1,231.1	1,094.3
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	383.0	380.7	1,149.3	1,075.1
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	39.3	58.5	149.4	159.2
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	383.4	316.5	1,027.5	932.1
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	236.9	186.5	616.7	541.7
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	146.5	130.0	410.8	390.4
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	215.5	247.4	703.6	654.2
Reportable segments | South America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	104.3	121.1	343.6	314.9
Reportable segments | South America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	107.9	123.1	350.6	329.1
Reportable segments | South America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	3.3	3.2	9.4	10.2
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	253.5	264.8	798.7	742.3
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	88.9	81.9	270.8	237.7
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	128.6	127.6	387.9	355.6
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	36.0	55.3	140.0	149.0
Other items not allocated to segments | Venezuela
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	0.0	20.8	51.4	115.2
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	11.6	18.4	60.0
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	9.2	33.0	55.2
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