Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net revenue	$ 1,895.7	$ 1,832.5	$ 5,617.9	$ 5,342.4
Billable expenses	401.8	375.7	1,240.5	1,115.4
Total revenue	2,297.5	2,208.2	6,858.4	6,457.8
OPERATING EXPENSES:
Salaries and related expenses	1,251.4	1,218.8	3,874.6	3,699.4
Office and other direct expenses	317.0	302.9	974.1	934.0
Billable expenses	401.8	375.7	1,240.5	1,115.4
Cost of services	1,970.2	1,897.4	6,089.2	5,748.8
Selling, general and administrative expenses	21.6	13.6	85.5	69.1
Depreciation and amortization	44.0	42.2	134.0	124.5
Total operating expenses	2,035.8	1,953.2	6,308.7	5,942.4
OPERATING INCOME	261.7	255.0	549.7	515.4
EXPENSES AND OTHER INCOME:
Interest expense	(27.6)	(21.0)	(73.6)	(67.6)
Interest income	5.3	4.1	14.0	14.0
Other expense, net	(15.3)	(9.9)	(56.0)	(24.5)
Total (expenses) and other income	(37.6)	(26.8)	(115.6)	(78.1)
Income before income taxes	224.1	228.2	434.1	437.3
Provision for income taxes	60.7	54.9	137.0	136.2
Income of consolidated companies	163.4	173.3	297.1	301.1
Equity in net income (loss) of unconsolidated affiliates	0.1	(1.0)	(1.9)	0.1
NET INCOME	163.5	172.3	295.2	301.2
Net (income) loss attributable to noncontrolling interests	(2.5)	(2.6)	(2.5)	0.9
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS	$ 161.0	$ 169.7	$ 292.7	$ 302.1
Earnings per share, Basic	$ 0.42	$ 0.44	$ 0.76	$ 0.77
Earnings per share, Diluted	$ 0.41	$ 0.43	$ 0.75	$ 0.76
Weighted-average number of common shares outstanding, Basic	382.6	389.5	383.2	391.2
Weighted-average number of common shares outstanding, Diluted	388.4	397.2	388.4	398.6
Dividends declared per common share	$ 0.21	$ 0.18	$ 0.63	$ 0.54