Revenue Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
NET INCOME	$ 972.8	$ 354.2	$ 673.9
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	283.8	290.6	278.5
Provision for uncollectible receivables	(14.6)	59.5	10.1
Amortization of restricted stock and other non-cash compensation	70.1	67.0	80.2
Net amortization of bond discounts and deferred financing costs	5.7	11.4	9.3
Deferred income tax provision	(8.2)	(46.4)	9.7
Net losses on sales of businesses	19.4	67.0	43.4
Other	18.7	20.0	6.8
Net Cash Provided by (Used in) Investing Activities [Abstract]
Accounts receivable	(647.6)	551.4	(111.2)
Accounts receivable, billable to clients	(571.0)	122.5	(38.7)
Other current assets	(45.9)	11.9	(27.2)
Accounts payable	1,858.1	(27.0)	546.0
Accrued liabilities	108.9	179.3	27.4
Contract liabilities	40.9	62.0	46.5
Other non-current assets and liabilities	(99.3)	(141.8)	(37.2)
Net cash provided by operating activities	2,075.6	1,847.2	1,529.2
Net cash used in investing activities	(185.3)	(216.2)	(161.7)
Net cash used in financing activities	(1,084.2)	(346.2)	(843.0)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	(45.4)	31.0	(6.0)
Net increase in cash, cash equivalents and restricted cash	760.7	1,315.8	518.5
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 3,272.2	$ 2,511.5	$ 1,195.7	$ 677.2