Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Net revenue	$ 2,413.7	$ 1,895.7	$ 1,948.2	$ 1,774.0	$ 2,131.1	$ 1,832.5	$ 1,834.6	$ 1,675.3	$ 8,031.6	$ 7,473.5	$ 7,452.3
Billable expenses	442.3	401.8	443.6	395.1	458.7	375.7	351.2	388.5	1,682.8	1,574.1	1,603.9
Total revenue	2,856.0	2,297.5	2,391.8	2,169.1	2,589.8	2,208.2	2,185.8	2,063.8	9,714.4	9,047.6	9,056.2
OPERATING EXPENSES:
Salaries and related expenses	1,423.7	1,251.4	1,292.9	1,330.3	1,291.3	1,218.8	1,228.9	1,251.7	5,298.3	4,990.7	4,942.2
Office and other direct expenses	381.0	317.0	333.3	323.8	334.8	302.9	318.4	312.7	1,355.1	1,268.8	1,274.9
Billable expense	442.3	401.8	443.6	395.1	458.7	375.7	351.2	388.5	1,682.8	1,574.1	1,603.9
Cost of services	2,247.0	1,970.2	2,069.8	2,049.2	2,084.8	1,897.4	1,898.5	1,952.9	8,336.2	7,833.6	7,821.0
Selling, General and Administrative Expense	81.0	21.6	28.8	35.1	49.4	13.6	20.3	35.2	166.5	118.5	138.6
Depreciation and amortization	68.9	44.0	44.0	46.0	32.6	42.2	41.3	41.0	202.9	157.1	160.2
Total operating expenses	2,396.9	2,035.8	2,142.6	2,130.3	2,166.8	1,953.2	1,960.1	2,029.1	8,705.6	8,109.2	8,119.8
OPERATING INCOME	459.1	261.7	249.2	38.8	423.0	255.0	225.7	34.7	1,008.8	938.4	936.4
EXPENSES AND OTHER INCOME:
Interest expense									(123.0)	(90.8)	(90.6)
Interest income									21.8	19.4	20.1
Other expense, net	(13.6)	(15.3)	(16.3)	(24.4)	(1.7)	(9.9)	(15.4)	0.8	(69.6)	(26.2)	(40.3)
Total (expenses) and other income	(55.2)	(37.6)	(37.7)	(40.3)	(19.5)	(26.8)	(36.4)	(14.9)	(170.8)	(97.6)	(110.8)
Income before income taxes									838.0	840.8	825.6
Provision for income taxes	62.2	60.7	63.6	12.7	135.1	54.9	81.6	(0.3)	199.2	271.3	196.9
Income of consolidated companies									638.8	569.5	628.7
Equity in net (loss) income of unconsolidated affiliates									(1.1)	0.9	0.3
NET INCOME	$ 342.5	$ 163.5	$ 147.8	$ (16.1)	$ 269.2	$ 172.3	$ 107.6	$ 21.3	637.7	570.4	629.0
Net income attributable to noncontrolling interests									(18.8)	(16.0)	(24.0)
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS									$ 618.9	$ 554.4	$ 605.0
Earnings per share available to IPG common stockholders:
Earnings per share, Basic	$ 0.85	$ 0.42	$ 0.38	$ (0.04)	$ 0.66	$ 0.44	$ 0.27	$ 0.06	$ 1.61	$ 1.42	$ 1.52
Earnings per share, Diluted	$ 0.84	$ 0.41	$ 0.37	$ (0.04)	$ 0.64	$ 0.43	$ 0.27	$ 0.06	$ 1.59	$ 1.40	$ 1.48
Weighted-average number of common shares outstanding:
Weighted-average number of common shares outstanding, Basic									383.3	389.6	397.9
Weighted-average number of common shares outstanding, Diluted									389.0	397.3	408.0