Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net Income Attributable to IPG Common Stockholders	$ 74.9	$ 89.7	$ 194.3	$ 168.2
Weighted Average Number of Shares Outstanding, Basic	407.6	419.2	409.7	421.0
Dilutive Securities, Effect on Basic Earnings Per Share, Options and Restrictive Stock Units	$ 7.9	$ 7.2	$ 7.3	$ 6.2
Weighted Average Number of Shares Outstanding, Diluted	415.5	426.4	417.0	427.2
Earnings Per Share, Basic	$ 0.18	$ 0.21	$ 0.47	$ 0.40
Earnings Per Share, Diluted	$ 0.18	$ 0.21	$ 0.47	$ 0.39