Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 977.7	$ 872.8
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	977.7	872.8
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	505.5	456.6
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	426.0	381.8
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	46.2	34.4
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	317.1	340.9
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	179.4	194.2
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	137.7	146.7
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 | Latin America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	269.7	299.0
Reportable segments | Latin America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	160.2	180.4
Reportable segments | Latin America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	105.5	114.9
Reportable segments | Latin America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	4.0	3.7
Reportable segments | Europe
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	214.4	126.3
Reportable segments | Europe | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	111.3	54.3
Reportable segments | Europe | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	71.1	41.8
Reportable segments | Europe | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	32.0	30.2
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	176.5	106.6
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	54.6	27.7
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	111.7	78.4
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 10.2	$ 0.5