Revenue Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 2,509.0	$ 1,192.2		$ 13.3
Accounts receivable, net of allowance of $98.3 and $40.2, respectively	4,646.4	5,209.2		112.9
Accounts receivable, billable to clients	1,820.7	1,934.1		8.3
Assets held for sale	0.8	22.8
Other current assets	390.7	412.4		28.2
Assets, Current	9,367.6	8,770.7
Deferred income taxes	302.0	252.1
Property, Plant and Equipment, Net	690.3	778.1		159.7
Goodwill	4,945.5	4,894.4	$ 4,875.9	1,110.8
Other intangible assets	933.6	1,014.3
Other non-current assets	424.4	467.9		8.3
Assets	18,042.7	17,751.9
Accounts Payable, Current	7,269.7	7,205.4
Accrued Liabilities, Current	832.4	742.8
Contract liabilities	657.8	585.6		23.2
Short-term borrowings	48.0	52.4
Current portion of long-term debt	502.5	502.0
Liabilities held for sale	1.6	65.0
Liabilities, Current	9,580.5	9,420.4
Long-term debt	2,915.8	2,771.9
Deferred compensation	413.2	425.0
Other non-current liabilities	655.2	714.7		$ 0.8
Liabilities	15,005.7	14,761.6
Redeemable noncontrolling interests	93.1	164.7	167.9		$ 252.1
STOCKHOLDERS' EQUITY:
Common stock, $0.10 par value, shares authorized: 800.0 shares issued: 2020 - 390.9; 2019 - 387.0 shares outstanding: 2020 - 390.9; 2019 - 387.0	39.0	38.7
Additional Paid in Capital	1,099.3	977.3
Retained Earnings (Accumulated Deficit)	2,636.9	2,689.9
Accumulated other comprehensive loss, net of tax	(880.2)	(930.0)	(941.1)
Stockholders' Equity Attributable to Parent	2,895.0	2,775.9
Stockholders' Equity Attributable to Noncontrolling Interest	48.9	49.7
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,943.9	2,825.6	$ 2,432.8		$ 2,246.3
Liabilities and Equity	$ 18,042.7	$ 17,751.9