Segment Operations (Details) - USD ($) $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segments:
Revenue	$ 2,341.0		$ 1,902.6		$ 1,884.9		$ 1,753.9		$ 2,264.5		$ 1,922.2		$ 1,917.9		$ 1,742.0		$ 7,882.4	$ 7,846.6	$ 7,613.8
Segment operating income																	973.6	941.0	874.8
Interest expense																	(90.8)	(90.6)	(85.8)
Interest income																	19.4	20.1	22.8
Other expense, net	(1.7)	[1],[2]	$ (9.9)	[1],[2]	$ (15.4)	[1],[2]	$ 0.8	[1],[2]	(26.8)	[1],[2]	$ 5.3	[1],[2]	$ 0.4	[1],[2]	$ (19.2)	[1],[2]	(26.2)	(40.3)	(49.6)
Income before income taxes																	876.0	830.2	762.2
Depreciation and amortization of fixed assets and intangible assets																	157.1	160.2	157.0
Capital expenditures																	155.9	200.7	161.1
Assets	12,695.2								12,485.2								12,695.2	12,485.2
IAN
Segments:
Revenue																	6,397.3	6,319.4	6,145.4
Segment operating income																	907.4	898.2	850.3
Depreciation and amortization of fixed assets and intangible assets																	111.8	117.7	117.5
Capital expenditures																	112.0	149.2	117.5
Assets	10,973.0								10,660.0								10,973.0	10,660.0
CMG
Segments:
Revenue																	1,485.1	1,527.2	1,468.4
Segment operating income																	192.8	190.0	166.3
Depreciation and amortization of fixed assets and intangible assets																	20.4	19.5	18.4
Capital expenditures																	17.7	16.6	12.7
Assets	1,422.9								1,428.3								1,422.9	1,428.3
Corporate and Other
Segments:
Segment operating income																	(126.6)	(147.2)	(141.8)
Depreciation and amortization of fixed assets and intangible assets																	24.9	23.0	21.1
Capital expenditures																	26.2	34.9	$ 30.9
Assets	$ 299.3								$ 396.9								$ 299.3	$ 396.9

[1] As part of the adoption of FASB ASU 2017-07, we have reclassified a portion of postretirement costs from salaries and related expenses to other income (expense), net.

[2] The three months ended June 30 and September 30, 2017 included pre-tax net losses of $13.1 and $8.7, respectively, on sales of businesses. The three months ended March 31 and December 31, 2016 included pre-tax net losses of $16.3 and $25.3, respectively, on sales of businesses.