Revenue Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 3,270.0	$ 2,509.0
Accounts receivable, net of allowance of $68.5 and $98.3, respectively	5,177.7	4,646.4
Accounts receivable, billable to clients	2,347.2	1,820.7
Assets held for sale	8.2	0.8
Other current assets	428.7	390.7
Assets, Current	11,231.8	9,367.6
Deferred income taxes	301.4	302.0
Property, Plant and Equipment, Net	675.8	690.3
Goodwill	4,908.7	4,945.5	$ 4,894.4
Other intangible assets	847.5	933.6
Other non-current assets	399.6	424.4
Assets	19,909.2	18,042.7
Accounts Payable, Current	8,960.0	7,269.7
Accrued Liabilities, Current	918.1	832.4
Contract liabilities	688.5	657.8
Short-term borrowings	47.5	48.0
Current portion of long-term debt	0.7	502.5
Liabilities held for sale	9.4	1.6
Liabilities, Current	10,890.0	9,580.5
Long-term debt	2,908.6	2,915.8
Deferred compensation	329.1	413.2
Other non-current liabilities	600.7	655.2
Liabilities	16,304.4	15,005.7
Redeemable noncontrolling interests	15.6	93.1	164.7	$ 167.9
STOCKHOLDERS' EQUITY:
Common stock, $0.10 par value, shares authorized: 800.0 shares issued: 2021 - 394.3; 2020 - 390.9 shares outstanding: 2021 - 394.3; 2020 - 390.9	39.3	39.0
Additional Paid in Capital	1,226.6	1,099.3
Retained Earnings (Accumulated Deficit)	3,154.3	2,636.9
Accumulated other comprehensive loss, net of tax	(894.2)	(880.2)	(930.0)
Stockholders' Equity Attributable to Parent	3,526.0	2,895.0
Stockholders' Equity Attributable to Noncontrolling Interest	63.2	48.9
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,589.2	2,943.9	$ 2,825.6	$ 2,432.8
Liabilities and Equity	$ 19,909.2	$ 18,042.7