Segment information - Customer Revenue by Classes of Similar Products or Services (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from External Customer [Line Items]
Revenue	$ 576.9	$ 495.2	$ 438.8	$ 515.4	$ 546.8	$ 552.1	$ 569.4	$ 554.5	$ 2,026.3	$ 2,222.8	$ 2,251.2
Services
Revenue from External Customer [Line Items]
Revenue									1,692.9	1,892.7	1,857.6
Technology
Revenue from External Customer [Line Items]
Revenue									333.4	330.1	393.6
Cloud & infrastructure services | Services
Revenue from External Customer [Line Items]
Revenue									1,178.1	1,287.2	1,225.4
Application services | Services
Revenue from External Customer [Line Items]
Revenue									350.2	370.9	381.7
BPO solutions | Services
Revenue from External Customer [Line Items]
Revenue									$ 164.6	$ 234.6	$ 250.5