Revenue Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
NET INCOME	$ 1,118.1	$ 956.1	$ 972.8
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	264.3	274.0	283.8
Provision for uncollectible receivables	7.2	(8.9)	(14.6)
Amortization of restricted stock and other non-cash compensation	46.7	50.0	70.1
Net amortization of bond discounts and deferred financing costs	2.0	3.0	5.7
Deferred income tax provision	5.1	(27.0)	(8.2)
Net (gains) losses on sales of businesses	(17.9)	11.3	19.4
Other	26.8	13.4	18.7
Net Cash Provided by (Used in) Investing Activities [Abstract]
Accounts receivable	(385.3)	(362.7)	(647.6)
Accounts receivable, billable to clients	(181.4)	244.5	(571.0)
Accounts payable	47.8	(408.0)	1,858.1
Accrued liabilities	(63.6)	(102.8)	108.9
Contract liabilities	(5.4)	17.8	40.9
Other non-current assets and liabilities	(220.6)	(59.3)	(99.3)
Net cash provided by operating activities	554.7	642.1	2,075.6
Net cash used in investing activities	(85.4)	(430.1)	(185.3)
Net cash used in financing activities	(634.3)	(899.4)	(1,084.2)
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	7.0	(31.7)	(45.4)
Net (decrease) increase in cash, cash equivalents and restricted cash	(158.0)	(719.1)	760.7
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 2,395.1	$ 2,553.1	$ 3,272.2	$ 2,511.5