Earnings Per Share Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Net income available to IPG common stockholders - basic		$ 260.3	$ 74.9	$ 121.2	$ (1.8)	$ 308.9	[1]	$ 89.7	$ 99.4	$ (20.9)	$ 454.6	$ 477.1	$ 259.2
Interest on convertible debt	[2]										0.0	0.0	0.8
Net income available to IPG common stockholders - diluted											$ 454.6	$ 477.1	$ 260.0
Weighted-average number of common shares outstanding - basic											408.1	419.2	421.1
Restricted stock, stock options and other equity awards											7.6	6.2	5.2
Convertible debt securities	[2]										0.0	0.0	3.3
Weighted-average number of common shares outstanding - diluted											415.7	425.4	429.6
Earnings per share available to IPG common stockholders - basic		$ 0.65	$ 0.18	$ 0.30	$ 0.00	$ 0.75		$ 0.21	$ 0.24	$ (0.05)	$ 1.11	$ 1.14	$ 0.62
Earnings per share available to IPG common stockholders - diluted		$ 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, 2014 included a tax benefit of $67.6 due to the net reversal of valuation allowances on deferred tax assets in Continental Europe.

[2] We retired all of our outstanding 4.75% Convertible Senior Notes due 2023 (the "4.75% Notes") in March 2013. For purposes of calculating diluted earnings per share for 2013, the potentially dilutive shares are pro-rated based on the period in which they were outstanding.