Revenue from contracts with customers - disaggregation of revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 1,074.0	$ 977.7
Reportable segments
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	1,074.0	977.7
Reportable segments | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	516.6	505.5
Reportable segments | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	505.7	426.0
Reportable segments | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	51.7	46.2
Reportable segments | North America
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	368.8	317.1
Reportable segments | North America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	185.6	179.4
Reportable segments | North America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	183.2	137.7
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	291.3	269.7
Reportable segments | Latin America | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	172.4	160.2
Reportable segments | Latin America | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	113.5	105.5
Reportable segments | Latin America | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	5.4	4.0
Reportable segments | Europe
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	222.1	214.4
Reportable segments | Europe | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	103.9	111.3
Reportable segments | Europe | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	84.7	71.1
Reportable segments | Europe | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	33.5	32.0
Reportable segments | Rest of World
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	191.8	176.5
Reportable segments | Rest of World | Core services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	54.7	54.6
Reportable segments | Rest of World | High-value services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	124.3	111.7
Reportable segments | Rest of World | Other security services
Disaggregation of Revenue [Line Items]
Revenue disaggregated by reportable segment and type of service	$ 12.8	$ 10.2