Results by Quarter (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended																12 Months Ended
		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenue		$ 2,264.5		$ 1,922.2		$ 1,917.9		$ 1,742.0		$ 2,196.2		$ 1,865.5		$ 1,876.1		$ 1,676.0		$ 7,846.6	$ 7,613.8	$ 7,537.1
Salaries and related expenses		1,309.4		1,228.8		1,229.0		1,270.9		1,235.1		1,202.2		1,205.2		1,215.2		5,038.1	4,857.7	4,820.4
Office and general expenses		470.0		486.2		464.1		450.2		504.7		471.4		455.1		453.0		1,870.5	1,884.2	1,928.3
Operating income		485.1		207.2		224.8		20.9		456.4		191.9		215.8		7.8		(938.0)	(871.9)	(788.4)
Other (expense) income, net		(26.2)	[1]	6.1	[1]	(0.1)	[1]	(17.1)	[1]	(10.3)	[1]	(37.2)	[1]	0.5	[1]	0.3	[1]	37.3	46.7	10.2
Total (expenses) and other income		(44.0)	[1]	(10.9)	[1]	(19.0)	[1]	(33.9)	[1]	(28.6)	[1]	(52.9)	[1]	(14.8)	[1]	(13.4)	[1]	107.8	109.7	67.7
Provision for (benefit of) income taxes		106.1	[2]	63.8	[2]	43.7	[2]	(15.6)	[2]	145.4	[2]	61.1	[2]	77.7	[2]	(1.4)	[2]	198.0	282.8	216.5
Net income (loss)		336.9		132.7		160.2		2.7		282.9		78.0		123.8		(4.2)		$ (632.5)	$ (480.5)	$ (505.4)
Net (loss) income available to IPG common stockholders	[2]	$ 317.6		$ 128.6		$ 156.9		$ 5.4		$ 260.3		$ 74.9		$ 121.2		$ (1.8)
Basic earnings per share		$ 0.81		$ 0.32		$ 0.39		$ 0.01		$ 0.65		$ 0.18		$ 0.30		$ 0.00		$ 1.53	$ 1.11	$ 1.14
Diluted earnings per share		0.78		0.32		0.38		0.01		0.63		0.18		0.29		0.00		1.49	1.09	1.12
Dividends declared per common share		$ 0.150		$ 0.150		$ 0.150		$ 0.150		$ 0.120		$ 0.120		$ 0.120		$ 0.120		$ 0.60	$ 0.48	$ 0.38
Pre-tax losses on sales of businesses		$ 25.3						$ 16.3		$ 12.0		$ 38.0
Tax benefit (Reversal of valuation allowance)								12.2										$ 12.2		$ 66.0
Tax benefit (Adoption of FASB ASU 2016-09)								$ 7.5										10.4
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 returns)		$ 37.4																$ 26.6	$ 13.0	$ 21.3

[1] The three months ended March 31 and December 31, 2016 included pre-tax losses of $16.3 and $25.3, respectively, on sales of businesses. The three months ended September 30 and December 31, 2015 included pre-tax losses of $38.0 and $12.0, respectively, on sales of businesses.

[2] The three months ended March 31, 2016 included tax benefits of $12.2 and $7.5 due to the reversal of valuation allowances and the adoption of FASB ASU 2016-09, respectively. The three months ended June 30 and December 31, 2016 included tax benefits of $23.4 related to the conclusion and settlement of a tax examination of previous years and $37.4 for refunds to be claimed on future amended U.S. federal returns, respectively.