Revenue Income Statement (Details) - USD ($) $ / shares in Units, 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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
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 (Loss)	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, 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, Basic									383.3	389.6	397.9
Weighted-average number of common shares outstanding, Diluted									389.0	397.3	408.0
As Previously Reported [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue										$ 7,508.7	$ 7,456.9
Billable expenses										373.7	389.7
Total revenue										7,882.4	7,846.6
OPERATING EXPENSES:
Salaries and related expenses										4,990.7	4,942.2
Office and other direct expenses										1,268.8	1,274.9
Billable expense										373.7	389.7
Cost of services										6,633.2	6,606.8
Selling, General and Administrative Expense										118.5	138.6
Depreciation and amortization										157.1	160.2
Total operating expenses										6,908.8	6,905.6
Operating Income (Loss)										973.6	941.0
EXPENSES AND OTHER INCOME:
Interest Expense										(90.8)	(90.6)
Interest income										19.4	20.1
Other expense, net										(26.2)	(40.3)
Total (expenses) and other income										(97.6)	(110.8)
Income before income taxes										876.0	830.2
Provision for income taxes										281.9	198.0
Income of consolidated companies										594.1	632.2
Equity in net (loss) income of unconsolidated affiliates										0.9	0.3
NET INCOME										595.0	632.5
Net income attributable to noncontrolling interests										(16.0)	(24.0)
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS										$ 579.0	$ 608.5
Earnings per share, Basic										$ 1.49	$ 1.53
Earnings per share, Diluted										$ 1.46	$ 1.49
Weighted-average number of common shares outstanding, Basic										389.6	397.9
Weighted-average number of common shares outstanding, Diluted										397.3	408.0
ASC 606 Adjustments [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue										$ (35.2)	$ (4.6)
Billable expenses										1,200.4	1,214.2
Total revenue										1,165.2	1,209.6
OPERATING EXPENSES:
Salaries and related expenses										0.0	0.0
Office and other direct expenses										0.0	0.0
Billable expense										1,200.4	1,214.2
Cost of services										1,200.4	1,214.2
Selling, General and Administrative Expense										0.0	0.0
Depreciation and amortization										0.0	0.0
Total operating expenses										1,200.4	1,214.2
Operating Income (Loss)										(35.2)	(4.6)
EXPENSES AND OTHER INCOME:
Interest Expense										0.0	0.0
Interest income										0.0	0.0
Other expense, net										0.0	0.0
Total (expenses) and other income										0.0	0.0
Income before income taxes										(35.2)	(4.6)
Provision for income taxes										(10.6)	(1.1)
Income of consolidated companies										(24.6)	(3.5)
Equity in net (loss) income of unconsolidated affiliates										0.0	0.0
NET INCOME										(24.6)	(3.5)
Net income attributable to noncontrolling interests										0.0	0.0
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS										$ (24.6)	$ (3.5)
Earnings per share, Basic										$ (0.07)	$ (0.01)
Earnings per share, Diluted										$ (0.06)	$ (0.01)
Weighted-average number of common shares outstanding, Basic										0.0	0.0
Weighted-average number of common shares outstanding, Diluted										0.0	0.0