Financial Assets and Liabilities - Effect of Hedge Activity on Income and Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Cost of services	$ 4,182.0	$ 4,332.0	$ 12,727.0	$ 12,876.0
Selling, general and administrative expenses	854.0	654.0	2,421.0	2,123.0
Other (income) and expense	(17.0)	8.0	16.0	17.0
Cost of services.
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) from Hedge Activity	2.0	3.0	11.0	5.0
Selling, general and administrative expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) from Hedge Activity		(2.0)	(6.0)	2.0
Other (income) and expense
Derivative Instruments, Gain (Loss) [Line Items]
Gains (Losses) from Hedge Activity	$ 5.0	$ 1.0	$ (3.0)	$ 10.0