Results by Quarter (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended											12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013		Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 2,207.1		$ 1,841.1	$ 1,851.4		$ 1,637.5	$ 2,122.7	$ 1,700.4		$ 1,756.2	$ 1,543.0	$ 7,537.1	$ 7,122.3	$ 6,956.2
Salaries and related expenses	1,266.4		1,195.2	1,170.2		1,188.6	1,199.6	1,093.6		1,120.2	1,132.1	4,820.4	4,545.5	4,391.9
Office and general expenses	507.6		474.7	484.7		461.1	538.1	465.5		460.9	453.4	1,928.1	1,917.9	1,887.2
Restructuring and other reorganization-related charges (reversals), net	0.1		(0.1)	0.7		(0.5)	60.6	(0.2)		0.3	(0.1)	0.2	60.6	(1.2)
Operating (loss) income	433.0		171.3	195.8		(11.7)	324.4	141.5		174.8	(42.4)	(788.4)	(598.3)	(678.3)
Other income (expense), net 1	(0.1)		(0.6)	(11.2)	[1]	1.7	7.7	(46.6)	[1]	4.8	1.8	10.2	32.3	(100.5)
Total (expenses) and other income	(14.4)		(13.8)	(27.2)	[1]	(12.3)	(10.3)	(64.5)	[1]	(26.9)	(28.6)	67.7	130.3	3.5
(Benefit of) provision for income taxes	87.9	[2]	65.0	65.3		(1.7)	103.2	28.4		62.0	(12.4)	216.5	181.2	213.3
Net (loss) income	(331.3)		(92.8)	(103.7)		22.4	(212.1)	(49.2)		(86.1)	58.5	(505.4)	(288.9)	(464.6)
Net (loss) income available to IPG common stockholders	308.9		89.7	99.4		(20.9)	193.1	45.4		79.9	(59.2)	477.1	259.2	435.1
Quarterly Information Tables [Line Items]
Gains (Losses) on Extinguishment of Debt												10.4	45.2	0
Dividends declared per common share	$ 0.095		$ 0.095	$ 0.095		$ 0.095	$ 0.075	$ 0.075		$ 0.075	$ 0.075	$ 0.38	$ 0.30	$ 0.24
Basic	$ 0.75		$ 0.21	$ 0.24		$ (0.05)	$ 0.45	$ 0.11		$ 0.19	$ (0.14)	$ 1.14	$ 0.62	$ 1.01
Diluted	$ 0.73		$ 0.21	$ 0.23		$ (0.05)	$ 0.44	$ 0.11		$ 0.18	$ (0.14)	$ 1.12	$ 0.61	$ 0.94
6.25% Notes
Quarterly Information Tables [Line Items]
Gains (Losses) on Extinguishment of Debt												$ 10.4

[1] The three months ended June 30, 2014 included a pre-tax loss of $10.4, related to our early extinguishment of debt. The three months ended September 30, 2013 included a pre-tax loss of $45.2, 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.