Revenue Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2023	Jan. 01, 2023	Dec. 31, 2022	Jan. 01, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 2,386.1		$ 2,545.3
Accounts receivable, net of allowance of $46.4 and $48.6, respectively	5,768.8		5,316.0
Accounts receivable, billable to clients	2,229.2		2,023.0
Assets held for sale	21.9		5.9
Other current assets	128.6		83.7
Assets, Current	10,950.4		10,325.2
Property, Plant and Equipment, Net	636.7		637.4
Goodwill	5,080.9	$ 5,050.6	5,050.6	$ 4,908.7	$ 4,908.7
Other intangible assets	743.6		818.1
Other non-current assets	428.1		431.2
Assets	19,267.3		18,811.7
Accounts Payable, Current	8,355.0		8,235.3
Accrued Liabilities, Current	705.8		787.1
Contract liabilities	684.7		680.0
Short-term borrowings	34.2		44.3
Current portion of long-term debt	250.1		0.6
Liabilities held for sale	48.5		0.0
Liabilities, Current	10,330.9		9,983.2
Long-term debt	2,917.5		2,870.7
Deferred compensation	223.6		294.1
Other non-current liabilities	532.4		572.6
Liabilities	15,221.2		15,100.7
Redeemable noncontrolling interests	42.3		38.3		15.6	$ 93.1
STOCKHOLDERS' EQUITY:
Common stock, $0.10 par value, shares authorized: 800.0 shares issued: 2023 - 383.0; 2022 - 389.6 shares outstanding: 2023 - 378.7; 2022 - 386.5	38.3		38.9
Additional Paid in Capital	728.5		1,057.5
Retained Earnings (Accumulated Deficit)	4,254.5		3,632.1
Accumulated other comprehensive loss, net of tax	(946.2)		(993.7)		(894.2)
Stockholders' Equity Attributable to Parent	3,942.6		3,614.6
Stockholders' Equity Attributable to Noncontrolling Interest	61.2		58.1
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	4,003.8		3,672.7		$ 3,589.2	$ 2,943.9
Liabilities and Equity	$ 19,267.3		$ 18,811.7