Revenue Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 1,192.2	$ 673.4
Accounts receivable, net of allowance of $40.2 and $42.5, respectively	5,209.2	5,126.6
Accounts receivable, billable to clients	1,934.1	1,900.6
Assets held for sale	22.8	5.7
Other current assets	412.4	476.6
Assets, Current	8,770.7	8,182.9
Deferred income taxes	252.1	247.0
Property, Plant and Equipment, Net	778.1	790.9
Goodwill	4,894.4	4,875.9	$ 3,820.4
Other intangible assets	1,014.3	1,094.7
Other non-current assets	467.9	428.9
Assets	17,751.9	15,620.3
Accounts Payable, Current	7,205.4	6,698.1
Accrued Liabilities, Current	742.8	806.9
Contract liabilities	585.6	533.9
Short-term borrowings	52.4	73.7
Current portion of long-term debt	502.0	0.1
Liabilities held for sale	65.0	11.2
Liabilities, Current	9,420.4	8,123.9
Long-term debt	2,771.9	3,660.2
Deferred compensation	425.0	422.7
Other non-current liabilities	714.7	812.8
Liabilities	14,761.6	13,019.6
Redeemable noncontrolling interests	164.7	167.9	252.1	$ 252.8
STOCKHOLDERS' EQUITY:
Common stock, $0.10 par value, shares authorized: 800.0 shares issued: 2019 - 387.0; 2018 - 383.6 shares outstanding: 2019 - 387.0; 2018 - 383.6	38.7	38.3
Additional Paid in Capital	977.3	895.9
Retained Earnings (Accumulated Deficit)	2,689.9	2,400.1
Accumulated other comprehensive loss, net of tax	(930.0)	(941.1)	(827.8)
Stockholders' Equity Attributable to Parent	2,775.9	2,393.2
Stockholders' Equity Attributable to Noncontrolling Interest	49.7	39.6
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,825.6	2,432.8	$ 2,246.3	$ 2,090.4
Liabilities and Equity	$ 17,751.9	$ 15,620.3