Segment information - Revenue, Properties and Outsourcing Assets by Geographic Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 760.9	$ 688.3	$ 667.4	$ 708.4	$ 744.8	$ 666.3	$ 666.2	$ 664.5	$ 2,825.0	$ 2,741.8	$ 2,820.7
Properties, net	121.3				142.5				121.3	142.5	145.3
Outsourcing assets, net	216.4				202.3				216.4	202.3	172.5
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,240.0	1,257.0	1,309.3
Properties, net	85.3				85.8				85.3	85.8	91.4
Outsourcing assets, net	97.6				81.1				97.6	81.1	105.1
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									360.7	315.8	348.0
Properties, net	5.3				16.7				5.3	16.7	15.1
Outsourcing assets, net	86.5				89.9				86.5	89.9	39.0
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
Outsourcing assets, net	21.7				18.1				21.7	18.1	11.5
Other foreign
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,224.3	1,169.0	1,163.4
Properties, net	30.7				40.0				30.7	40.0	38.8
Outsourcing assets, net	$ 10.6				$ 13.2				$ 10.6	$ 13.2	$ 16.9