Segment Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segments:
Total revenue	$ 2,856.0	$ 2,297.5	$ 2,391.8	$ 2,169.1	$ 2,589.8	$ 2,208.2	$ 2,185.8	$ 2,063.8	$ 9,714.4	$ 9,047.6	$ 9,056.2
Net revenue	2,413.7	1,895.7	1,948.2	1,774.0	2,131.1	1,832.5	1,834.6	1,675.3	8,031.6	7,473.5	7,452.3
Operating Income (Loss)	459.1	261.7	249.2	38.8	423.0	255.0	225.7	34.7	1,008.8	938.4	936.4
Interest expense									(123.0)	(90.8)	(90.6)
Interest income									21.8	19.4	20.1
Other expense, net	(13.6)	(15.3)	(16.3)	(24.4)	(1.7)	(9.9)	(15.4)	0.8	(69.6)	(26.2)	(40.3)
Income before income taxes									838.0	840.8	825.6
Depreciation and amortization	68.9	$ 44.0	$ 44.0	$ 46.0	32.6	$ 42.2	$ 41.3	$ 41.0	202.9	157.1	160.2
Capital expenditures									177.1	155.9	200.7
Assets	15,620.3				12,704.7				15,620.3	12,704.7
IAN
Segments:
Total revenue									7,374.4	7,009.6	6,992.8
Net revenue									6,585.8	6,266.7	6,201.4
Operating Income (Loss)									982.8	875.1	894.3
Depreciation and amortization									142.8	125.5	129.2
Capital expenditures									128.5	112.0	149.2
Assets	11,446.2				10,978.0				11,446.2	10,978.0
CMG
Segments:
Total revenue									2,158.3	2,038.0	2,063.4
Net revenue									1,264.1	1,206.8	1,250.9
Operating Income (Loss)									175.0	189.9	189.3
Depreciation and amortization									24.3	23.5	22.4
Capital expenditures									13.0	17.7	16.6
Assets	1,516.7				1,427.4				1,516.7	1,427.4
Corporate and Other
Segments:
Total revenue									181.7	0.0	0.0
Net revenue									181.7	0.0	0.0
Operating Income (Loss)									(149.0)	(126.6)	(147.2)
Depreciation and amortization									35.8	8.1	8.6
Capital expenditures									35.6	26.2	$ 34.9
Assets	$ 2,657.4				$ 299.3				$ 2,657.4	$ 299.3