Revenue Income Statement (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	$ 1,774.0	$ 1,675.3
Billable expense	395.1	388.5
Total revenue	2,169.1	2,063.8
OPERATING EXPENSES:
Salaries and related expenses	1,330.3	1,251.7
Office and other direct expenses	323.8	312.7
Billable expenses	395.1	388.5
Cost of services	2,049.2	1,952.9
Selling, general and administrative expenses	35.1	35.2
Depreciation and amortization	46.0	41.0
Total operating expenses	2,130.3	2,029.1
OPERATING INCOME	38.8	34.7
EXPENSES AND OTHER INCOME:
Interest expense	(19.9)	(20.9)
Interest income	4.0	5.2
Other income, net	(24.4)	0.8
Total (expenses) and other income	(40.3)	(14.9)
Income before income taxes	(1.5)	19.8
Benefit of income taxes	12.7	(0.3)
(Loss) income of consolidated companies	(14.2)	20.1
Equity in net income of unconsolidated affiliates	(1.9)	1.2
NET (LOSS) INCOME	(16.1)	21.3
Net loss attributable to noncontrolling interests	2.0	3.4
NET (LOSS) INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS	$ (14.1)	$ 24.7
Earnings per share, Basic	$ (0.04)	$ 0.06
Earnings per share, Diluted	$ (0.04)	$ 0.06
Weighted-average number of common shares outstanding, Basic	383.4	391.7
Weighted-average number of common shares outstanding, Diluted	383.4	399.3
Dividends declared per common share	$ 0.21	$ 0.18
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue		$ 1,670.3
Billable expense		83.6
Total revenue		1,753.9
OPERATING EXPENSES:
Salaries and related expenses		1,251.7
Office and other direct expenses		312.7
Billable expenses		83.6
Cost of services		1,648.0
Selling, general and administrative expenses		35.2
Depreciation and amortization		41.0
Total operating expenses		1,724.2
OPERATING INCOME		29.7
EXPENSES AND OTHER INCOME:
Interest expense		(20.9)
Interest income		5.2
Other income, net		0.8
Total (expenses) and other income		(14.9)
Income before income taxes		14.8
Benefit of income taxes		(2.1)
(Loss) income of consolidated companies		16.9
Equity in net income of unconsolidated affiliates		1.2
NET (LOSS) INCOME		18.1
Net loss attributable to noncontrolling interests		3.4
NET (LOSS) INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS		$ 21.5
Earnings per share, Basic		$ 0.05
Earnings per share, Diluted		$ 0.05
Weighted-average number of common shares outstanding, Basic		391.7
Weighted-average number of common shares outstanding, Diluted		399.3
Dividends declared per common share		$ 0.18
ASC 606 Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue		$ 5.0
Billable expense		304.9
Total revenue		309.9
OPERATING EXPENSES:
Billable expenses		304.9
Cost of services		304.9
Total operating expenses		304.9
OPERATING INCOME		5.0
EXPENSES AND OTHER INCOME:
Income before income taxes		5.0
Benefit of income taxes		1.8
(Loss) income of consolidated companies		3.2
NET (LOSS) INCOME		3.2
NET (LOSS) INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS		$ 3.2
Earnings per share, Basic		$ 0.01
Earnings per share, Diluted		$ 0.01