Revenue Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 163.5	$ 172.3	$ 295.2	$ 301.2
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	44.0	42.2	134.0	124.5
Provision for uncollectible receivables			9.4	9.5
Amortization of restricted stock and other non-cash compensation			59.7	59.8
Net amortization of bond discounts and deferred financing costs			4.2	4.2
Deferred income tax benefit			(22.9)	18.8
Net gains on sales of businesses	5.8	8.7	50.0	20.9
Other			4.2	16.1
Changes in assets and liabilities, net of acquisitions and divestitures, providing (using) cash:
Accounts receivable			443.6	875.8
Expenditures billable to clients				0.0
Accounts receivable, billable to clients			(303.0)	(166.9)
Other current assets			(100.6)	(63.9)
Accounts payable			(766.9)	(961.7)
Accrued liabilities			(186.5)	(287.8)
Contract liabilities			46.1	(68.1)
Other non-current assets and liabilities			6.8	(21.4)
Net cash used in operating activities			(326.7)	(139.0)
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures	(44.2)	(39.8)	(105.7)	(108.7)
Acquisitions, net of cash acquired			(12.0)	(22.6)
Other investing activities			1.1	(9.2)
Net cash used in investing activities			(116.6)	(140.5)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase in short-term borrowings			(4.6)	429.9
Exercise of stock options			9.1	12.1
Common stock dividends			(241.6)	(211.2)
Repurchase of common stock			(117.1)	(216.0)
Tax payments for employee shares withheld			(28.8)	(38.4)
Distributions to noncontrolling interests			(13.4)	(16.9)
Other financing activities			(11.9)	0.1
Net cash provided by (used in) financing activities			1,550.6	(113.1)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash			(35.3)	0.4
Net decrease in cash, cash equivalents and restricted cash			1,072.0	(392.2)
Cash, cash equivalents, and restricted cash at beginning of period			797.7	1,100.2
Cash, cash equivalents, and restricted cash at end of period	$ 1,869.7	708.0	$ 1,869.7	708.0
As Previously Reported
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income		148.8		261.5
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization				124.5
Provision for uncollectible receivables				9.5
Amortization of restricted stock and other non-cash compensation				59.8
Net amortization of bond discounts and deferred financing costs				4.2
Deferred income tax benefit				(1.6)
Net gains on sales of businesses				20.9
Other				16.1
Changes in assets and liabilities, net of acquisitions and divestitures, providing (using) cash:
Accounts receivable				875.8
Expenditures billable to clients				(165.9)
Accounts receivable, billable to clients				0.0
Other current assets				(48.2)
Accounts payable				(986.4)
Accrued liabilities				(287.8)
Contract liabilities				0.0
Other non-current assets and liabilities				(21.4)
Net cash used in operating activities				(139.0)
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash used in investing activities				(140.5)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net cash provided by (used in) financing activities				(113.1)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash				0.4
Net decrease in cash, cash equivalents and restricted cash				(392.2)
Cash, cash equivalents, and restricted cash at beginning of period				1,100.2
Cash, cash equivalents, and restricted cash at end of period		708.0		708.0
ASC 606 Adjustments | Accounting Standards Update 2014-09
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income		23.5		39.7
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization				0.0
Provision for uncollectible receivables				0.0
Amortization of restricted stock and other non-cash compensation				0.0
Net amortization of bond discounts and deferred financing costs				0.0
Deferred income tax benefit				20.4
Net gains on sales of businesses				0.0
Other				0.0
Changes in assets and liabilities, net of acquisitions and divestitures, providing (using) cash:
Accounts receivable				0.0
Expenditures billable to clients				165.9
Accounts receivable, billable to clients				(166.9)
Other current assets				(15.7)
Accounts payable				24.7
Accrued liabilities				0.0
Contract liabilities				(68.1)
Other non-current assets and liabilities				0.0
Net cash used in operating activities				0.0
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash used in investing activities				0.0
CASH FLOWS FROM FINANCING ACTIVITIES:
Net cash provided by (used in) financing activities				0.0
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash				0.0
Net decrease in cash, cash equivalents and restricted cash				0.0
Cash, cash equivalents, and restricted cash at beginning of period				0.0
Cash, cash equivalents, and restricted cash at end of period		$ 0.0		$ 0.0