Revenue Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 673.4	$ 790.9
Accounts receivable, net of allowance	5,126.6	4,585.0
Expenditures billable to clients		0.0
Accounts receivable, billable to clients	1,900.6	1,747.4
Assets held for sale	5.7	5.7
Other current assets	476.6	346.5
Assets, Current	8,182.9	7,475.5
Deferred income taxes	247.0	234.0
Property, Plant and Equipment, Net	790.9	650.4
Goodwill	4,875.9	3,820.4	$ 3,674.4
Other intangible assets	1,094.7	140.7
Other non-current assets	428.9	383.7
Assets	15,620.3	12,704.7
Accounts Payable, Current	6,698.1	6,420.2
Accrued Liabilities, Current	806.9	674.7
Contract liabilities	533.9	484.7
Short-term borrowings	73.7	84.9
Current portion of long-term debt	0.1	2.0
Liabilities held for sale	11.2	8.8
Liabilities, Current	8,123.9	7,675.3
Long-term debt	3,660.2	1,285.6
Deferred compensation	422.7	476.6
Other non-current liabilities	812.8	768.8
Liabilities	13,019.6	10,206.3
Redeemable noncontrolling interests	167.9	252.1	252.8	$ 251.9
STOCKHOLDERS' EQUITY:
Common Stock, Value, Issued	38.3	38.6
Additional Paid in Capital	895.9	955.2
Retained Earnings (Accumulated Deficit)	2,400.1	2,104.5
Accumulated other comprehensive loss, net of tax	(941.1)	(827.8)	(964.4)
Stockholders' Equity before Treasury Stock	2,393.2	2,270.5
Treasury Stock, Value	0.0	59.0
Stockholders' Equity Attributable to Parent	2,393.2	2,211.5
Stockholders' Equity Attributable to Noncontrolling Interest	39.6	34.8
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,432.8	2,246.3	2,090.4	$ 2,001.8
Liabilities and Equity	$ 15,620.3	12,704.7
As Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents		790.9
Accounts receivable, net of allowance		4,585.0
Expenditures billable to clients		1,747.4
Accounts receivable, billable to clients		0.0
Assets held for sale		5.7
Other current assets		335.1
Assets, Current		7,464.1
Deferred income taxes		236.0
Property, Plant and Equipment, Net		650.4
Goodwill		3,820.4
Other non-current assets		383.6
Assets		12,695.2
Accounts Payable, Current		6,907.8
Accrued Liabilities, Current		674.7
Contract liabilities		0.0
Short-term borrowings		84.9
Current portion of long-term debt		2.0
Liabilities held for sale		8.8
Liabilities, Current		7,678.2
Long-term debt		1,285.6
Deferred compensation		476.6
Other non-current liabilities		766.9
Liabilities		10,207.3
Redeemable noncontrolling interests		252.1
STOCKHOLDERS' EQUITY:
Common Stock, Value, Issued		38.6
Additional Paid in Capital		955.2
Retained Earnings (Accumulated Deficit)		2,093.6
Accumulated other comprehensive loss, net of tax		(827.4)
Stockholders' Equity before Treasury Stock		2,260.0
Treasury Stock, Value		59.0
Stockholders' Equity Attributable to Parent		2,201.0
Stockholders' Equity Attributable to Noncontrolling Interest		34.8
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		2,235.8
Liabilities and Equity		12,695.2
ASC 606 Adjustments [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents		0.0
Accounts receivable, net of allowance		0.0
Expenditures billable to clients		(1,747.4)
Accounts receivable, billable to clients		1,747.4
Assets held for sale		0.0
Other current assets		11.4
Assets, Current		11.4
Deferred income taxes		(2.0)
Property, Plant and Equipment, Net		0.0
Goodwill		0.0
Other intangible assets		0.0
Other non-current assets		0.1
Assets		9.5
Accounts Payable, Current		(487.6)
Accrued Liabilities, Current		0.0
Contract liabilities		484.7
Short-term borrowings		0.0
Current portion of long-term debt		0.0
Liabilities held for sale		0.0
Liabilities, Current		(2.9)
Long-term debt		0.0
Deferred compensation		0.0
Other non-current liabilities		1.9
Liabilities		(1.0)
Redeemable noncontrolling interests		0.0
STOCKHOLDERS' EQUITY:
Common Stock, Value, Issued		0.0
Additional Paid in Capital		0.0
Retained Earnings (Accumulated Deficit)		10.9	35.6
Accumulated other comprehensive loss, net of tax		(0.4)	$ (1.9)
Stockholders' Equity before Treasury Stock		10.5
Treasury Stock, Value		0.0
Stockholders' Equity Attributable to Parent		10.5
Stockholders' Equity Attributable to Noncontrolling Interest		0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		10.5
Liabilities and Equity		$ 9.5