Results by Quarter (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [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
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
Acxiom Acquisition Costs					22.6	11.0	1.4
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 (Loss)	491.3	280.3	264.2	50.2	459.1	261.7	249.2	38.8	1,086.0	1,008.8	938.4
Net losses on sales of businesses	23.9	7.7	3.2	8.6	11.9	5.8	19.8	24.4	(43.4)	(61.9)	(24.1)
Tax Adjustments, Settlements, and Unusual Provisions			13.9
reversal of valuation allowance	25.3
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)
Various Discrete Tax Items					23.4
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 available to IPG common stockholders	$ 328.9	$ 165.6	$ 169.5	$ (8.0)	$ 326.2	$ 161.0	$ 145.8	$ (14.1)
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
Dividends declared per common share	$ 0.235	$ 0.235	$ 0.235	$ 0.235	$ 0.210	$ 0.210	$ 0.210	$ 0.210