Segment Data - Schedule of reconciliation of Fee-based operating expenses by segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Fee-based operating expenses	$ 1,497.5	$ 1,346.0	$ 4,233.6	$ 3,894.9
Depreciation and amortization	71.6	64.1	213.0	193.0
Integration and other costs related to acquisitions	71.5	71.2	175.8	200.3
Pre-IPO stock-based compensation	10.8	6.2	25.7	20.5
Cassidy Turley deferred payment obligation	11.0	10.2	32.3	32.3
Other	3.0	3.4	7.6	11.5
Operating segments
Segment Reporting Information [Line Items]
Fee-based operating expenses	1,329.6	1,190.9	3,779.2	3,437.3
Operating segments | Americas
Segment Reporting Information [Line Items]
Fee-based operating expenses	921.7	806.0	2,577.9	2,337.9
Operating segments | EMEA
Segment Reporting Information [Line Items]
Fee-based operating expenses	182.9	165.8	537.1	466.0
Operating segments | APAC
Segment Reporting Information [Line Items]
Fee-based operating expenses	$ 225.0	$ 219.1	$ 664.2	$ 633.4