Revenue Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
NET INCOME	$ 342.5	$ 163.5	$ 147.8	$ (16.1)	$ 269.2	$ 172.3	$ 107.6	$ 21.3	$ 637.7	$ 570.4	$ 629.0
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									202.9	157.1	160.2
Provision for uncollectible receivables									6.5	9.5	16.7
Amortization of restricted stock and other non-cash compensation									82.2	82.0	85.6
Net amortization of bond discounts and deferred financing costs									6.5	5.8	5.6
Deferred income tax provision									14.1	(9.5)	44.6
Net losses on sales of businesses									61.9	24.1	41.4
Other									1.2	12.7	35.5
Net Cash Provided by (Used in) Investing Activities [Abstract]
Accounts receivable									(603.8)	37.6	(220.7)
Expenditures billable to clients										0.0	0.0
Accounts receivable, billable to clients									(209.5)	(165.5)	(2.2)
Other current assets									(67.2)	50.1	0.5
Accounts payable									428.7	336.4	(147.4)
Accrued liabilities									(24.2)	(241.3)	(61.1)
Contract liabilities									44.9	(12.0)	20.6
Other non-current assets and liabilities									(16.8)	24.4	(95.5)
Net cash provided by operating activities									565.1	881.8	512.8
Net cash used in investing activities									(2,491.5)	(196.2)	(263.9)
Net cash provided by (used in) financing activities									1,853.2	(1,004.9)	(666.4)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash									(47.3)	16.8	11.6
Net decrease in cash, cash equivalents and restricted cash									(120.5)	(302.5)	(405.9)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 677.2				797.7				$ 677.2	797.7	1,100.2	$ 1,506.1
As Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
NET INCOME										595.0	632.5
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization										157.1	160.2
Provision for uncollectible receivables										9.5	16.7
Amortization of restricted stock and other non-cash compensation										82.0	85.6
Net amortization of bond discounts and deferred financing costs										5.8	5.6
Deferred income tax provision										1.1	45.7
Net losses on sales of businesses										24.1	41.4
Other										12.7	35.5
Net Cash Provided by (Used in) Investing Activities [Abstract]
Accounts receivable										37.6	(220.7)
Expenditures billable to clients										(165.5)	(2.2)
Accounts receivable, billable to clients										0.0	0.0
Other current assets										27.4	(4.8)
Accounts payable										311.9	(126.1)
Accrued liabilities										(241.3)	(61.1)
Contract liabilities										0.0	0.0
Other non-current assets and liabilities										24.4	(95.5)
Net cash provided by operating activities										881.8	512.8
Net cash used in investing activities										(196.2)	(263.9)
Net cash provided by (used in) financing activities										(1,004.9)	(666.4)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash										16.8	11.6
Net decrease in cash, cash equivalents and restricted cash										(302.5)	(405.9)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents					797.7					797.7	1,100.2	1,506.1
ASC 606 Adjustments [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
NET INCOME										(24.6)	(3.5)
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization										0.0	0.0
Provision for uncollectible receivables										0.0	0.0
Amortization of restricted stock and other non-cash compensation										0.0	0.0
Net amortization of bond discounts and deferred financing costs										0.0	0.0
Deferred income tax provision										(10.6)	(1.1)
Net losses on sales of businesses										0.0	0.0
Other										0.0	0.0
Net Cash Provided by (Used in) Investing Activities [Abstract]
Accounts receivable										0.0	0.0
Expenditures billable to clients										165.5	2.2
Accounts receivable, billable to clients										(165.5)	(2.2)
Other current assets										22.7	5.3
Accounts payable										24.5	(21.3)
Accrued liabilities										0.0	0.0
Contract liabilities										(12.0)	20.6
Other non-current assets and liabilities										0.0	0.0
Net cash provided by operating activities										0.0	0.0
Net cash used in investing activities										0.0	0.0
Net cash provided by (used in) financing activities										0.0	0.0
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash										0.0	0.0
Net decrease in cash, cash equivalents and restricted cash										0.0	0.0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents					$ 0.0					$ 0.0	$ 0.0	$ 0.0