Results by Quarter (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
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
Salaries and related expenses	1,325.8	[1]	1,227.6	[1]	1,239.3	[1]	1,275.4	[1]	1,308.8	[1]	1,228.0	[1]	1,229.5	[1]	1,268.8	[1]	5,068.1	5,035.1	4,854.8
Office and general expenses	496.9		455.9		439.1		448.8		470.0		486.2		464.1		450.2		1,840.7	1,870.5	1,884.2
Operating income	518.3		219.1		206.5		29.7		485.7		208.0		224.3		23.0		973.6	941.0	874.8
Other income (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)
Total (expenses) and other income	(19.5)		(26.8)		(36.4)		(14.9)		(44.6)		(11.7)		(18.5)		(36.0)		(97.6)	(110.8)	(112.6)
(Benefit of) provision for income taxes	166.1	[3]	42.5	[3]	75.4	[3]	(2.1)	[3]	106.1	[3]	63.8	[3]	43.7	[3]	(15.6)	[3]	281.9	198.0	282.8
Net income	333.5		148.8		94.6		18.1		336.9		132.7		160.2		2.7		$ 595.0	$ 632.5	$ 480.5
Net income available to IPG common stockholders	$ 316.6		$ 146.2		$ 94.7		$ 21.5		$ 317.6		$ 128.6		$ 156.9		$ 5.4
Basic earnings per share	$ 0.82		$ 0.38		$ 0.24		$ 0.05		$ 0.81		$ 0.32		$ 0.39		$ 0.01		$ 1.49	$ 1.53	$ 1.11
Diluted earnings per share	0.81		0.37		0.24		0.05		0.78		0.32		0.38		0.01		1.46	1.49	1.09
Dividends declared per common share	$ 0.18		$ 0.18		$ 0.18		$ 0.18		$ 0.15		$ 0.15		$ 0.15		$ 0.15		$ 0.72	$ 0.60	$ 0.48
Pre-tax losses on sales of businesses			$ 8.7		$ 13.1				$ 25.3						$ 16.3
Foreign tax credits			$ 31.2
Net impact of the Tax Act																	$ 36.0	$ 0.0	$ 0.0
Tax benefit (reversal of valuation allowance)																		12.2
Tax benefit (conclusion and settlement of a tax examination of previous years)																		$ 23.4
Tax benefit (refunds to be claimed on future amended U.S. federal tax returns)									$ 37.4

[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.

[3] The three months ended September 30, 2017 included a tax benefit of $31.2 related to foreign tax credits from distributions of unremitted earnings, which was reversed during the three months ended December 31, 2017 as a result of the enactment of the Tax Cuts and Jobs Act. The three months ended December 31, 2017 included a net tax benefit of $36.0 as a result of the enactment of the Tax Cuts and Jobs Act. The three months ended March 31, 2016 included a tax benefit of $12.2 due to the reversal of valuation allowances. The three months ended June 30, 2016 included a tax benefit of $23.4 related to the conclusion and settlement of a tax examination of previous years. The three months ended December 31, 2016 included a tax benefit of $37.4 for refunds to be claimed on future amended U.S. federal returns.