Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income available to IPG common stockholders	$ 161.0	$ 169.7	$ 292.7	$ 302.1
Weighted-average number of common shares outstanding, Basic	382.6	389.5	383.2	391.2
Dilutive effect of stock options and restricted shares	5.8	7.7	5.2	7.4
Weighted-average number of common shares outstanding, Diluted	388.4	397.2	388.4	398.6
Earnings per share, Basic	$ 0.42	$ 0.44	$ 0.76	$ 0.77
Earnings per share, Diluted	$ 0.41	$ 0.43	$ 0.75	$ 0.76