Segment Information - Customer Revenue by Classes of Similar Products or Services (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Revenues	$ 688.3	$ 666.3	$ 2,064.1	$ 1,997.0
Services
Revenue from External Customer [Line Items]
Revenues	605.6	575.5	1,760.8	1,735.6
Technology
Revenue from External Customer [Line Items]
Revenues	82.7	90.8	303.3	261.4
Cloud & infrastructure services | Services
Revenue from External Customer [Line Items]
Revenues	348.3	326.9	1,000.2	994.8
Application services | Services
Revenue from External Customer [Line Items]
Revenues	191.2	200.7	578.1	592.9
Business process outsourcing services | Services
Revenue from External Customer [Line Items]
Revenues	$ 66.1	$ 47.9	$ 182.5	$ 147.9