Segment Information Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segments:
Revenue	$ 1,902.6	$ 1,922.2	$ 5,541.4	$ 5,582.1
Segment operating income (loss):	219.1	208.0	455.3	455.3
Interest expense	(21.0)	(21.7)	(67.6)	(68.8)
Interest income	4.1	4.7	14.0	16.1
Other (expense) income, net	(9.9)	5.3	(24.5)	(13.5)
Income before income taxes	192.3	196.3	377.2	389.1
Depreciation and amortization of fixed assets and intangible assets	42.2	39.7	124.5	117.5
Capital expenditures	39.8	51.5	108.7	114.5
IAN
Segments:
Revenue	1,520.2	1,503.2	4,465.6	4,453.3
Segment operating income (loss):	183.9	184.1	402.1	424.1
Depreciation and amortization of fixed assets and intangible assets	30.7	29.1	90.8	85.9
Capital expenditures	29.3	39.7	77.9	86.9
CMG
Segments:
Revenue	382.4	419.0	1,075.8	1,128.8
Segment operating income (loss):	50.1	54.8	127.4	125.2
Depreciation and amortization of fixed assets and intangible assets	4.6	4.9	15.2	14.6
Capital expenditures	6.2	4.7	12.9	8.4
Corporate and Other
Segments:
Segment operating income (loss):	(14.9)	(30.9)	(74.2)	(94.0)
Depreciation and amortization of fixed assets and intangible assets	6.9	5.7	18.5	17.0
Capital expenditures	$ 4.3	$ 7.1	$ 17.9	$ 19.2