Revenue Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ (16.1)	$ 21.3
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	46.0	41.0
Provision for uncollectible receivables	2.1	5.9
Amortization of restricted stock and other non-cash compensation	30.0	29.7
Net amortization of bond discounts and deferred financing costs	1.4	1.4
Deferred income tax benefit	(20.8)	(12.0)
Net gains on sales of businesses	24.4	(0.9)
Other	6.8	6.7
Changes in assets and liabilities, net of acquisitions and divestitures, providing (using) cash:
Accounts receivable	675.2	806.6
Expenditures billable to clients		0.0
Accounts receivable, billable to clients	(220.7)	(206.5)
Other current assets	(88.3)	(68.6)
Accounts payable	(994.2)	(726.5)
Accrued liabilities	(164.6)	(264.9)
Contract liabilities	17.6	16.2
Other non-current assets and liabilities	(28.7)	(21.2)
Net cash used in operating activities	(729.9)	(371.8)
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures	(22.8)	(24.8)
Acquisitions, net of cash acquired	(0.2)	(3.3)
Other investing activities	(0.1)	(5.1)
Net cash used in investing activities	(23.1)	(33.2)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase in short-term borrowings	718.8	224.8
Exercise of stock options	6.9	8.2
Common stock dividends	(80.8)	(70.9)
Repurchase of common stock	(54.9)	(55.0)
Tax payments for employee shares withheld	(26.3)	(36.7)
Distributions to noncontrolling interests	(3.9)	(6.0)
Other financing activities	(1.6)	0.0
Net cash provided by financing activities	558.2	64.4
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	(2.9)	20.0
Net decrease in cash, cash equivalents and restricted cash	(197.7)	(320.6)
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	$ 600.0	779.6
As Previously Reported
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income		18.1
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization		41.0
Provision for uncollectible receivables		5.9
Amortization of restricted stock and other non-cash compensation		29.7
Net amortization of bond discounts and deferred financing costs		1.4
Deferred income tax benefit		(13.8)
Net gains on sales of businesses		(0.9)
Other		6.7
Changes in assets and liabilities, net of acquisitions and divestitures, providing (using) cash:
Accounts receivable		806.6
Expenditures billable to clients		(206.3)
Accounts receivable, billable to clients		0.0
Other current assets		(71.6)
Accounts payable		(702.5)
Accrued liabilities		(264.9)
Contract liabilities		0.0
Other non-current assets and liabilities		(21.2)
Net cash used in operating activities		(371.8)
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash used in investing activities		(33.2)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net cash provided by financing activities		64.4
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash		20.0
Net decrease in cash, cash equivalents and restricted cash		(320.6)
Cash, cash equivalents, and restricted cash at beginning of period		1,100.2
Cash, cash equivalents, and restricted cash at end of period		779.6
ASC 606 Adjustments
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income		3.2
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		1.8
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		206.3
Accounts receivable, billable to clients		(206.5)
Other current assets		3.0
Accounts payable		(24.0)
Accrued liabilities		0.0
Contract liabilities		16.2
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 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