Revenue Recognition - Narrative (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
Revenue from Contract with Customer [Abstract]
Payment terms									60 days
Description of timing									Our client contracts are generally short-term in nature with a term of one year or less. The timing between satisfaction of the performance obligation, invoicing and payment is not significant.
Disaggregation of Revenue [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
Deferred revenue	$ 42.8				$ 48.0				42.8	48.0
Franchise fees
Disaggregation of Revenue [Line Items]
Revenues from services									$ 24.1	$ 23.7	$ 23.3