Segment Data - Segment Information by Geographical Region (Details) - USD ($) $ 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
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services	$ 5,054.9	$ 4,584.8	$ 3,742.2	$ 4,619.1	$ 5,196.6	$ 5,248.9	$ 5,373.1	$ 5,044.9	$ 18,001.0	$ 20,863.5	$ 21,991.2
Long-Lived Assets	154.3				150.9				154.3	150.9	160.0
United States Plans
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									2,327.2	2,590.6	2,608.9
Long-Lived Assets	16.5				20.5				16.5	20.5	22.4
France
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									4,338.1	5,479.6	5,846.4
Long-Lived Assets	44.8				46.9				44.8	46.9	50.1
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									1,370.7	1,508.3	1,673.9
Long-Lived Assets	6.0				5.2				6.0	5.2	5.0
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									1,402.9	1,598.6	1,672.1
Long-Lived Assets	5.0				5.8				5.0	5.8	7.0
Total Foreign
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									15,673.8	18,272.9	19,382.3
Long-Lived Assets	$ 137.8				$ 130.4				$ 137.8	$ 130.4	$ 137.6