Segment Information Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segments:
Revenue	$ 1,922.2	$ 1,865.5	$ 5,582.1	$ 5,417.6
Segment operating income (loss):	207.2	191.9	452.9	415.5
Interest expense	(21.7)	(21.3)	(68.8)	(62.5)
Interest income	4.7	5.6	16.1	17.8
Other income (expense), net	6.1	(37.2)	(11.1)	(36.4)
Income before income taxes	196.3	139.0	389.1	334.4
Depreciation and amortization of fixed assets and intangible assets	39.7	38.1	117.5	116.3
Capital expenditures	51.5	30.9	114.5	80.7
IAN
Segments:
Revenue	1,503.2	1,484.1	4,453.3	4,351.3
Segment operating income (loss):	183.3	182.9	421.7	419.2
Depreciation and amortization of fixed assets and intangible assets	29.1	27.9	85.9	87.5
Capital expenditures	39.7	23.0	86.9	55.4
CMG
Segments:
Revenue	419.0	381.4	1,128.8	1,066.3
Segment operating income (loss):	54.8	48.2	125.2	109.5
Depreciation and amortization of fixed assets and intangible assets	4.9	4.9	14.6	13.9
Capital expenditures	4.7	2.3	8.4	5.8
Corporate and Other
Segments:
Segment operating income (loss):	(30.9)	(39.2)	(94.0)	(113.2)
Depreciation and amortization of fixed assets and intangible assets	5.7	5.3	17.0	14.9
Capital expenditures	$ 7.1	$ 5.6	$ 19.2	$ 19.5