Results by Quarter (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 2,196.2		$ 1,865.5		$ 1,876.1	$ 1,676.0	$ 2,207.1		$ 1,841.1	$ 1,851.4		$ 1,637.5	$ 7,613.8	$ 7,537.1	$ 7,122.3
Salaries and related expenses	1,235.1		1,202.2		1,205.2	1,215.2	1,266.4		1,195.2	1,170.2		1,188.6	4,857.7	4,820.4	4,545.5
Office and general expenses	504.7		472.1		455.1	453.1	507.6		474.7	484.7		461.1	1,885.0	1,928.1	1,917.9
Restructuring and other reorganization-related (reversals) charges, net	0.0		(0.7)		0.0	(0.1)	0.1		(0.1)	0.7		(0.5)	(0.8)	0.2	60.6
Operating income (loss)	456.4		191.9		215.8	7.8	433.0		171.3	195.8		(11.7)	(871.9)	(788.4)	(598.3)
Other income (expense), net	10.3	[1]	37.2	[1]	(0.5)	(0.3)	0.1		0.6	11.2	[1]	(1.7)	46.7	10.2	32.3
Total (expenses) and other income	(28.6)	[1]	(52.9)	[1]	(14.8)	(13.4)	(14.4)		13.8	(27.2)	[1]	(12.3)	109.7	67.7	130.3
(Benefit of) provision for income taxes	145.4		61.1		77.7	(1.4)	87.9	[2]	65.0	65.3		(1.7)	282.8	216.5	181.2
Net (loss) income	(282.9)		(78.0)		(123.8)	4.2	(331.3)		(92.8)	(103.7)		22.4	(480.5)	(505.4)	(288.9)
Net (loss) income available to IPG common stockholders	$ 260.3		$ 74.9		$ 121.2	$ (1.8)	$ 308.9	[2]	$ 89.7	$ 99.4		$ (20.9)	$ 454.6	$ 477.1	$ 259.2
Dividends declared per common share	$ 0.120		$ 0.120		$ 0.120	$ 0.120	$ 0.095		$ 0.095	$ 0.095		$ 0.095	$ 0.48	$ 0.38	$ 0.30
Basic earnings per share	0.65		0.18		0.30	0.00	0.75		0.21	0.24		(0.05)	1.11	1.14	0.62
Diluted earnings per share	$ 0.63		$ 0.18		$ 0.29	$ 0.00	$ 0.73		$ 0.21	$ 0.23		$ (0.05)	$ 1.09	$ 1.12	$ 0.61

[1] The three months ended December 31, 2015 included pre-tax losses of $12.0 on sales of businesses. The three months ended September 30, 2015 included pre-tax losses of $38.0 on sales of businesses. The three months ended June 30, 2014 included a pre-tax loss of $10.4 related to our early extinguishment of debt.

[2] The three months ended December 31, 2014 included a tax benefit of $67.6 due to the net reversal of valuation allowances on deferred tax assets in Continental Europe.