Goodwill (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Goodwill
Goodwill	$ 3,669.2		$ 3,629.0		$ 3,580.6	[1]
Current year acquisitions	136.1	[1]	75.3	[1]
Foreign currency and other	(95.9)	[1]	(26.9)	[1]
IAN
Goodwill
Goodwill	3,120.8	[1]	3,109.3	[1]	3,074.6	[1]
Current year acquisitions	95.2	[1]	58.8	[1]
Foreign currency and other	(83.7)	[1]	(24.1)	[1]
CMG
Goodwill
Goodwill	548.4	[1]	519.7	[1]	506.0	[1]
Current year acquisitions	40.9	[1]	16.5	[1]
Foreign currency and other	$ (12.2)	[1]	$ (2.8)	[1]

[1] For all periods presented we have not recorded a goodwill impairment charge.