Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net Income Attributable to IPG Common Stockholders	$ 121.2	$ 99.4	$ 119.4	$ 78.5
Weighted Average Number of Shares Outstanding, Basic	410.5	421.1	410.8	421.9
Dilutive Securities, Effect on Basic Earnings Per Share, Options and Restrictive Stock Units	$ 7.1	$ 7.0	$ 6.8	$ 6.6
Weighted Average Number of Shares Outstanding, Diluted	417.6	428.1	417.6	428.5
Earnings Per Share, Basic	$ 0.30	$ 0.24	$ 0.29	$ 0.19
Earnings Per Share, Diluted	$ 0.29	$ 0.23	$ 0.29	$ 0.18