Segment information - Revenue And Operating Profits (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 903.2	$ 849.5	$ 805.9	$ 788.4	$ 803.5	$ 755.8	$ 739.5	$ 721.8	$ 3,347.0	$ 3,020.6	$ 3,061.4
Operating Profit	$ 88.3	$ 66.4	$ 48.3	$ 70.9	$ 69.1	$ 59.7	$ 32.2	23.5	273.9	184.5	96.4
Foreign currency transaction losses									(9.2)	1.4	(26.8)
Reportable Segments:
Segment Reporting Information [Line Items]
Revenues									3,192.9	2,908.4	2,976.9
Operating Profit									372.0	274.0	252.7
Corporate items:
Segment Reporting Information [Line Items]
General, administrative and other expenses									(84.3)	(59.8)	(81.0)
Foreign currency transaction losses									(1.1)	3.8	(8.8)
Reconciliation of segment policies to GAAP									(5.2)	(2.2)	4.6
Other items not allocated to segments:
Segment Reporting Information [Line Items]
Acquisitions and dispositions, Revenues								$ 2.8
Reorganization and Restructuring									(22.6)	(30.3)	(15.3)
Acquisitions and dispositions, Operating Profit (Loss)									(5.3)	(19.5)	(10.2)
Venezuela
Segment Reporting Information [Line Items]
Revenues									154.1	109.4	84.5
Venezuela | Other items not allocated to segments:
Segment Reporting Information [Line Items]
Revenues									154.1	109.4	84.5
Operating Profit									20.4	18.5	(45.6)
North America | Reportable Segments:
Segment Reporting Information [Line Items]
Revenues									1,254.2	1,210.3	1,224.0
Operating Profit									74.0	40.1	45.6
South America | Reportable Segments:
Segment Reporting Information [Line Items]
Revenues									924.6	718.7	719.3
Operating Profit									182.8	122.6	104.6
Rest of World | Reportable Segments:
Segment Reporting Information [Line Items]
Revenues									1,014.1	979.4	1,033.6
Operating Profit									115.2	111.3	102.5
Core services
Segment Reporting Information [Line Items]
Revenues									1,719.5	1,597.6	1,615.7
High-value services
Segment Reporting Information [Line Items]
Revenues									1,409.8	1,209.3	1,208.8
Other security services
Segment Reporting Information [Line Items]
Revenues									$ 217.7	$ 213.7	$ 236.9