Results by Quarter (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [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
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 (Loss)	459.1	261.7	249.2	38.8	423.0	255.0	225.7	34.7	1,008.8	938.4	936.4
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)
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
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 available to IPG common stockholders	$ 326.2	$ 161.0	$ 145.8	$ (14.1)	$ 252.3	$ 169.7	$ 107.7	$ 24.7
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
Dividends declared per common share	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.18	$ 0.18	$ 0.18	$ 0.18