Goodwill (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill
Goodwill		$ 3,820.4	$ 3,674.4	$ 3,608.5	[1]
Current year acquisitions		55.1	122.0
Foreign currency and other		90.9	(56.1)
IAN
Goodwill
Goodwill	[1]	3,161.8	3,043.8	3,051.4
Current year acquisitions		39.6	32.7
Foreign currency and other		78.4	(40.3)
CMG
Goodwill
Goodwill	[1]	658.6	630.6	$ 557.1
Current year acquisitions		15.5	89.3
Foreign currency and other		$ 12.5	$ (15.8)

[1] For all periods presented, no goodwill impairment charge has been recorded.