Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Net revenue	$ 2,433.0	$ 2,061.4	$ 2,125.9	$ 2,004.8	$ 2,413.7	$ 1,895.7	$ 1,948.2	$ 1,774.0	$ 8,625.1	$ 8,031.6	$ 7,473.5
Billable expenses	468.8	376.7	394.3	356.4	442.3	401.8	443.6	395.1	1,596.2	1,682.8	1,574.1
Total revenue	2,901.8	2,438.1	2,520.2	2,361.2	2,856.0	2,297.5	2,391.8	2,169.1	10,221.3	9,714.4	9,047.6
OPERATING EXPENSES:
Salaries and related expenses	1,432.1	1,334.4	1,381.2	1,421.1	1,423.7	1,251.4	1,292.9	1,330.3	5,568.8	5,298.3	4,990.7
Office and other direct expenses	419.7	367.9	387.3	389.2	381.0	317.0	333.3	323.8	1,564.1	1,355.1	1,269.2
Billable expense	468.8	376.7	394.3	356.4	442.3	401.8	443.6	395.1	1,596.2	1,682.8	1,574.1
Cost of services	2,320.6	2,079.0	2,162.8	2,166.7	2,247.0	1,970.2	2,069.8	2,049.2	8,729.1	8,336.2	7,834.0
Selling, General and Administrative Expense	24.5	9.8	18.1	41.4	81.0	21.6	28.8	35.1	93.8	166.5	118.5
Depreciation and amortization	65.4	69.0	73.0	71.1	68.9	44.0	44.0	46.0	278.5	202.9	157.1
Restructuring Charges	0.0	0.0	2.1	31.8	0.0	0.0	0.0	0.0	33.9	0.0	(0.4)
Total operating expenses	2,410.5	2,157.8	2,256.0	2,311.0	2,396.9	2,035.8	2,142.6	2,130.3	9,135.3	8,705.6	8,109.2
OPERATING INCOME	491.3	280.3	264.2	50.2	459.1	261.7	249.2	38.8	1,086.0	1,008.8	938.4
EXPENSES AND OTHER INCOME:
Interest expense									(199.3)	(123.0)	(90.8)
Interest income									34.5	21.8	19.4
Other expense, net	(24.8)	(7.4)	(3.8)	(6.9)	(13.6)	(15.3)	(16.3)	(24.4)	(42.9)	(69.6)	(26.2)
Total (expenses) and other income	(63.5)	(47.6)	(47.7)	(48.9)	(55.2)	(37.6)	(37.7)	(40.3)	(207.7)	(170.8)	(97.6)
Income before income taxes									878.3	838.0	840.8
Provision for income taxes	86.1	64.6	43.6	10.5	62.2	60.7	63.6	12.7	204.8	199.2	271.3
Income of consolidated companies									673.5	638.8	569.5
Equity in net (loss) income of unconsolidated affiliates									0.4	(1.1)	0.9
NET INCOME	$ 342.2	$ 168.4	$ 172.8	$ (9.5)	$ 342.5	$ 163.5	$ 147.8	$ (16.1)	673.9	637.7	570.4
Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest									(17.9)	(18.8)	(16.0)
Net Income (Loss) Attributable to Parent									$ 656.0	$ 618.9	$ 554.4
Earnings per share available to IPG common stockholders:
Earnings per share, Basic	$ 0.85	$ 0.43	$ 0.44	$ (0.02)	$ 0.85	$ 0.42	$ 0.38	$ (0.04)	$ 1.70	$ 1.61	$ 1.42
Earnings per share, Diluted	$ 0.84	$ 0.42	$ 0.43	$ (0.02)	$ 0.84	$ 0.41	$ 0.37	$ (0.04)	$ 1.68	$ 1.59	$ 1.40
Weighted-average number of common shares outstanding:
Weighted-average number of common shares outstanding, Basic									386.1	383.3	389.6
Weighted-average number of common shares outstanding, Diluted									391.2	389.0	397.3