Revenue Income Statement (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	$ 2,284.4	$ 1,954.6	$ 1,853.4	$ 1,972.1	$ 2,433.0	$ 2,061.4	$ 2,125.9	$ 2,004.8	$ 8,064.5	$ 8,625.1	$ 8,031.6
Billable expenses	265.6	170.9	172.3	387.7	468.8	376.7	394.3	356.4	996.5	1,596.2	1,682.8
Total revenue	2,550.0	2,125.5	2,025.7	2,359.8	2,901.8	2,438.1	2,520.2	2,361.2	9,061.0	10,221.3	9,714.4
OPERATING EXPENSES:
Salaries and related expenses	1,346.2	1,269.9	1,306.1	1,422.8	1,432.1	1,334.4	1,381.2	1,421.1	5,345.0	5,568.8	5,298.3
Office and other direct expenses	364.8	307.9	317.0	378.2	419.7	367.9	387.3	389.2	1,367.9	1,564.1	1,355.1
Billable expense	265.6	170.9	172.3	387.7	468.8	376.7	394.3	356.4	996.5	1,596.2	1,682.8
Cost of services	1,976.6	1,748.7	1,795.4	2,188.7	2,320.6	2,079.0	2,162.8	2,166.7	7,709.4	8,729.1	8,336.2
Selling, General and Administrative Expense	22.4	9.9	4.1	22.4	24.5	9.8	18.1	41.4	58.8	93.8	166.5
Depreciation and amortization	73.7	71.0	73.1	72.8	65.4	69.0	73.0	71.1	290.6	278.5	202.9
Total operating expenses	2,326.6	1,876.9	1,985.2	2,283.9	2,410.5	2,157.8	2,256.0	2,311.0	8,472.6	9,135.3	8,705.6
Operating Income (Loss)	223.4	248.6	40.5	75.9	491.3	280.3	264.2	50.2	588.4	1,086.0	1,008.8
EXPENSES AND OTHER INCOME:
Interest Expense									(192.2)	(199.3)	(123.0)
Interest income									29.5	34.5	21.8
Other expense, net	(9.8)	(11.3)	(21.5)	(21.8)	(24.8)	(7.4)	(3.8)	(6.9)	(64.4)	(42.9)	(69.6)
Total (expenses) and other income	(49.8)	(56.0)	(65.4)	(55.9)	(63.5)	(47.6)	(47.7)	(48.9)	(227.1)	(207.7)	(170.8)
Income before income taxes									361.3	878.3	838.0
Provision for income taxes	58.1	(86.3)	19.0	17.2	86.1	64.6	43.6	10.5	8.0	204.8	199.2
Income of consolidated companies									353.3	673.5	638.8
Equity in net (loss) income of unconsolidated affiliates									0.9	0.4	(1.1)
NET INCOME	$ 117.0	$ 278.5	$ (43.9)	$ 2.6	$ 342.2	$ 168.4	$ 172.8	$ (9.5)	354.2	673.9	637.7
Net Income (Loss) Attributable to Parent									$ 351.1	$ 656.0	$ 618.9
Earnings per share, Basic	$ 0.29	$ 0.72	$ (0.12)	$ 0.01	$ 0.85	$ 0.43	$ 0.44	$ (0.02)	$ 0.90	$ 1.70	$ 1.61
Earnings per share, Diluted	$ 0.28	$ 0.71	$ (0.12)	$ 0.01	$ 0.84	$ 0.42	$ 0.43	$ (0.02)	$ 0.89	$ 1.68	$ 1.59
Weighted-average number of common shares outstanding, Basic									389.4	386.1	383.3
Weighted-average number of common shares outstanding, Diluted									393.2	391.2	389.0