Revenue Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 147.8	$ 107.6	$ 131.7	$ 128.9
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	44.0	41.3	90.0	82.3
Provision for uncollectible receivables			6.1	8.6
Amortization of restricted stock and other non-cash compensation			46.0	46.0
Net amortization of bond discounts and deferred financing costs			2.7	2.8
Deferred income tax benefit			(31.0)	9.5
Net gains on sales of businesses	19.8	13.1	44.2	12.2
Other			1.9	10.2
Changes in assets and liabilities, net of acquisitions and divestitures, providing (using) cash:
Accounts receivable			238.0	769.7
Expenditures billable to clients				0.0
Accounts receivable, billable to clients			(233.7)	(165.5)
Other current assets			(124.6)	(84.5)
Accounts payable			(579.3)	(651.3)
Accrued liabilities			(175.9)	(313.9)
Contract liabilities			38.0	6.9
Other non-current assets and liabilities			(11.8)	(15.0)
Net cash used in operating activities			(557.7)	(153.1)
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures	(38.7)	(44.1)	(61.5)	(68.9)
Acquisitions, net of cash acquired			(8.5)	(12.6)
Other investing activities			12.4	(14.5)
Net cash used in investing activities			(57.6)	(96.0)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase in short-term borrowings			669.3	153.5
Exercise of stock options			7.0	11.8
Common stock dividends			(161.2)	(141.4)
Repurchase of common stock			(114.5)	(115.0)
Tax payments for employee shares withheld			(28.0)	(37.8)
Distributions to noncontrolling interests			(10.6)	(10.9)
Other financing activities			(0.3)	0.3
Net cash provided by (used in) financing activities			341.0	(199.4)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash			(27.5)	8.2
Net decrease in cash, cash equivalents and restricted cash			(301.8)	(440.3)
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	$ 495.9	659.9	$ 495.9	659.9
As Previously Reported
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income		94.6		112.7
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization				82.3
Provision for uncollectible receivables				8.6
Amortization of restricted stock and other non-cash compensation				46.0
Net amortization of bond discounts and deferred financing costs				2.8
Deferred income tax benefit				1.5
Net gains on sales of businesses				12.2
Other				10.2
Changes in assets and liabilities, net of acquisitions and divestitures, providing (using) cash:
Accounts receivable				769.7
Expenditures billable to clients				(165.3)
Accounts receivable, billable to clients				0.0
Other current assets				(82.0)
Accounts payable				(622.9)
Accrued liabilities				(313.9)
Contract liabilities				0.0
Other non-current assets and liabilities				(15.0)
Net cash used in operating activities				(153.1)
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash used in investing activities				(96.0)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net cash provided by (used in) financing activities				(199.4)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash				8.2
Net decrease in cash, cash equivalents and restricted cash				(440.3)
Cash, cash equivalents, and restricted cash at beginning of period				1,100.2
Cash, cash equivalents, and restricted cash at end of period		659.9		659.9
ASC 606 Adjustments
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income		13.0		16.2
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				8.0
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				165.3
Accounts receivable, billable to clients				(165.5)
Other current assets				(2.5)
Accounts payable				(28.4)
Accrued liabilities				0.0
Contract liabilities				6.9
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