Revenue Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
NET INCOME	$ 342.2	$ 168.4	$ 172.8	$ (9.5)	$ 342.5	$ 163.5	$ 147.8	$ (16.1)	$ 673.9	$ 637.7	$ 570.4
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									278.5	202.9	157.1
Provision for uncollectible receivables									10.1	6.5	9.5
Amortization of restricted stock and other non-cash compensation									80.2	82.2	82.0
Net amortization of bond discounts and deferred financing costs									9.3	6.5	5.8
Deferred income tax provision									9.7	14.1	(9.5)
Net losses on sales of businesses	(23.9)	$ (7.7)	$ (3.2)	$ (8.6)	(11.9)	$ (5.8)	$ (19.8)	$ (24.4)	43.4	61.9	24.1
Other									9.9	1.2	12.7
Net Cash Provided by (Used in) Investing Activities [Abstract]
Accounts receivable									(111.2)	(603.8)	37.6
Accounts receivable, billable to clients									(38.7)	(209.5)	(165.5)
Other current assets									(27.2)	(67.2)	50.1
Accounts payable									546.0	428.7	336.4
Accrued liabilities									27.4	(24.2)	(241.3)
Contract liabilities									46.5	44.9	(12.0)
Other non-current assets and liabilities									(29.3)	(16.8)	24.4
Net cash provided by operating activities									1,529.2	565.1	881.8
Net cash used in investing activities									(161.7)	(2,491.5)	(196.2)
Net cash (used in) provided by financing activities									(843.0)	1,853.2	(1,004.9)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash									(6.0)	(47.3)	16.8
Net increase (decrease) in cash, cash equivalents and restricted cash									518.5	(120.5)	(302.5)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 1,195.7				$ 677.2				$ 1,195.7	$ 677.2	$ 797.7	$ 1,100.2