Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Net income available to IPG common stockholders, Basic	$ 45.4		$ 68.7		$ 66.1		$ 121.8
Net income available to IPG common stockholders, Diluted	45.4		69.7		66.1		125.2
Weighted-average number of common shares outstanding, Basic	419.7		431.3		419.7		435.5
Weighted-average number of common shares outstanding, Diluted	426.1		456.1		424.8		469.7
Earnings per share available to IPG common stockholders, Basic	$ 0.11		$ 0.16		$ 0.16		$ 0.28
Earnings per share available to IPG common stockholders, Diluted	$ 0.11		$ 0.15		$ 0.16		$ 0.27
Restricted stock, stock options and other equity awards, Effect of dilutive securities, Incremental common shares	6.4		8.1		5.1		7.1
4.75% Notes
Interest on convertible debt, Effect of dilutive securities	0	[1]	1.0	[1]	0	[1]	3.1	[1]
Convertible debt, Effect of dilutive securities, Incremental common shares	0	[1]	16.7	[1]	0	[1]	16.7	[1]
4.25% Notes
Interest on convertible debt, Effect of dilutive securities	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0.3	[1]
Convertible debt, Effect of dilutive securities, Incremental common shares	0	[1]	0	[1]	0	[1]	10.4	[1]

[1] We retired all of our outstanding 4.75% Notes and 4.25% Convertible Senior Notes due 2023 (the “4.25% Notes”) in March 2013 and March 2012, respectively. See Note 2 for further information on our 4.75% Notes. For purposes of calculating diluted earnings per share, the potentially dilutive shares are pro-rated based on the period they were outstanding.