Revenue Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2022	Jan. 01, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 2,545.3		$ 3,270.0
Accounts receivable, net of allowance of $68.5 and $98.3, respectively	5,316.0		5,177.7
Accounts receivable, billable to clients	2,023.0		2,347.2
Assets held for sale	5.9		8.2
Other current assets	435.0		428.7
Assets, Current	10,325.2		11,231.8
Deferred income taxes	271.7		301.4
Property, Plant and Equipment, Net	637.4		675.8
Goodwill	5,050.6	$ 4,908.7	4,908.7	$ 4,945.5
Other intangible assets	818.1		847.5
Other non-current assets	464.5		399.6
Assets	18,845.0		19,909.2
Accounts Payable, Current	8,235.3		8,960.0
Accrued Liabilities, Current	787.1		918.1
Contract liabilities	680.0		688.5
Short-term borrowings	44.3		47.5
Current portion of long-term debt	0.6		0.7
Liabilities held for sale	0.0		9.4
Liabilities, Current	9,983.2		10,890.0
Long-term debt	2,870.7		2,908.6
Deferred compensation	294.1		329.1
Other non-current liabilities	572.6		600.7
Liabilities	15,100.7		16,304.4
Redeemable noncontrolling interests	38.3		15.6	93.1	$ 164.7
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	38.9		39.3
Additional Paid in Capital	1,057.5		1,226.6
Retained Earnings (Accumulated Deficit)	3,632.1		3,154.3
Accumulated other comprehensive loss, net of tax	(960.4)		(894.2)	(880.2)
Stockholders' Equity Attributable to Parent	3,647.9		3,526.0
Stockholders' Equity Attributable to Noncontrolling Interest	58.1		63.2
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3,706.0		3,589.2	$ 2,943.9	$ 2,825.6
Liabilities and Equity	$ 18,845.0		$ 19,909.2