Segment Information - Customer Revenue by Classes of Similar Products or Services (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue from External Customer [Line Items]
Revenues	$ 753.8	$ 667.4	$ 1,449.6	$ 1,375.8
Services
Revenue from External Customer [Line Items]
Revenues	653.4	586.7	1,265.5	1,155.2
Technology
Revenue from External Customer [Line Items]
Revenues	100.4	80.7	184.1	220.6
Cloud & infrastructure services | Services
Revenue from External Customer [Line Items]
Revenues	391.3	333.5	752.5	651.9
Application services | Services
Revenue from External Customer [Line Items]
Revenues	196.7	194.0	385.8	386.9
Business process outsourcing services | Services
Revenue from External Customer [Line Items]
Revenues	$ 65.4	$ 59.2	$ 127.2	$ 116.4