Segment Data - Segment Information by Geographical Region (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services	$ 5,393.2	$ 5,418.7	$ 5,656.9	$ 5,522.4	$ 5,637.5	$ 5,464.8	$ 5,174.8	$ 4,757.2	$ 21,991.2	$ 21,034.3	$ 19,654.1
Long-Lived Assets	160.0				161.3				160.0	161.3	150.6
Franchise fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									24.1	23.7	23.3
Americas
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									4,159.3	4,216.4	4,297.2
Americas | United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									2,522.3	2,659.0	2,836.8
Americas | United States | Franchise fees
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									15.0	14.8	15.1
Americas | Reportable segments
Segment Reporting, Revenue Reconciling Item [Line Items]
Long-Lived Assets	25.6				26.7				25.6	26.7	34.0
Americas | Reportable segments | United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Long-Lived Assets	19.6				20.6				19.6	20.6	27.7
United States Plans
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									2,608.9	2,758.5	2,950.2
Long-Lived Assets	22.4				24.9				22.4	24.9	33.9
France
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									5,846.4	5,493.9	4,857.3
Long-Lived Assets	50.1				49.1				50.1	49.1	40.9
Italy
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									1,673.9	1,479.4	1,170.7
Long-Lived Assets	5.0				4.9				5.0	4.9	4.4
United Kingdom
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									1,672.1	1,619.2	1,819.7
Long-Lived Assets	7.0				9.1				7.0	9.1	9.0
Total Foreign
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenues from Services									19,382.3	18,275.8	16,703.9
Long-Lived Assets	$ 137.6				$ 136.4				$ 137.6	$ 136.4	$ 116.7