Revenue Income Statement (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Revenue	$ 1,948.2	$ 1,834.6	$ 3,722.2	$ 3,509.9
Billable expenses	443.6	351.2	838.7	739.7
Total revenue	2,391.8	2,185.8	4,560.9	4,249.6
OPERATING EXPENSES:
Salaries and related expenses	1,292.9	1,228.9	2,623.2	2,480.6
Office and other direct expenses	333.3	318.4	657.1	631.1
Billable expenses	443.6	351.2	838.7	739.7
Cost of services	2,069.8	1,898.5	4,119.0	3,851.4
Selling, general and administrative expenses	28.8	20.3	63.9	55.5
Depreciation and amortization	44.0	41.3	90.0	82.3
Total operating expenses	2,142.6	1,960.1	4,272.9	3,989.2
OPERATING INCOME	249.2	225.7	288.0	260.4
EXPENSES AND OTHER INCOME:
Interest expense	(26.1)	(25.7)	(46.0)	(46.6)
Interest income	4.7	4.7	8.7	9.9
Other income, net	(16.3)	(15.4)	(40.7)	(14.6)
Total (expenses) and other income	(37.7)	(36.4)	(78.0)	(51.3)
Income before income taxes	211.5	189.3	210.0	209.1
Benefit of income taxes	63.6	81.6	76.3	81.3
Income of consolidated companies	147.9	107.7	133.7	127.8
Equity in net income of unconsolidated affiliates	(0.1)	(0.1)	(2.0)	1.1
NET INCOME	147.8	107.6	131.7	128.9
Net (income) loss attributable to noncontrolling interests	(2.0)	0.1	0.0	3.5
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS	$ 145.8	$ 107.7	$ 131.7	$ 132.4
Earnings per share, Basic	$ 0.38	$ 0.27	$ 0.34	$ 0.34
Earnings per share, Diluted	$ 0.37	$ 0.27	$ 0.34	$ 0.33
Weighted-average number of common shares outstanding, Basic	383.6	392.3	383.5	392.0
Weighted-average number of common shares outstanding, Diluted	389.5	400.3	388.9	399.6
Dividends declared per common share	$ 0.21	$ 0.18	$ 0.42	$ 0.36
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Revenue		$ 1,815.4		$ 3,485.7
Billable expenses		69.5		153.1
Total revenue		1,884.9		3,638.8
OPERATING EXPENSES:
Salaries and related expenses		1,228.9		2,480.6
Office and other direct expenses		318.4		631.1
Billable expenses		69.5		153.1
Cost of services		1,616.8		3,264.8
Selling, general and administrative expenses		20.3		55.5
Depreciation and amortization		41.3		82.3
Total operating expenses		1,678.4		3,402.6
OPERATING INCOME		206.5		236.2
EXPENSES AND OTHER INCOME:
Interest expense		(25.7)		(46.6)
Interest income		4.7		9.9
Other income, net		(15.4)		(14.6)
Total (expenses) and other income		(36.4)		(51.3)
Income before income taxes		170.1		184.9
Benefit of income taxes		75.4		73.3
Income of consolidated companies		94.7		111.6
Equity in net income of unconsolidated affiliates		(0.1)		1.1
NET INCOME		94.6		112.7
Net (income) loss attributable to noncontrolling interests		0.1		3.5
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS		$ 94.7		$ 116.2
Earnings per share, Basic		$ 0.24		$ 0.30
Earnings per share, Diluted		$ 0.24		$ 0.29
Weighted-average number of common shares outstanding, Basic		392.3		392.0
Weighted-average number of common shares outstanding, Diluted		400.3		399.6
Dividends declared per common share		$ 0.18		$ 0.36
ASC 606 Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Revenue		$ 19.2		$ 24.2
Billable expenses		281.7		586.6
Total revenue		300.9		610.8
OPERATING EXPENSES:
Salaries and related expenses		0.0		0.0
Office and other direct expenses		0.0		0.0
Billable expenses		281.7		586.6
Cost of services		281.7		586.6
Selling, general and administrative expenses		0.0		0.0
Depreciation and amortization		0.0		0.0
Total operating expenses		281.7		586.6
OPERATING INCOME		19.2		24.2
EXPENSES AND OTHER INCOME:
Interest expense		0.0		0.0
Interest income		0.0		0.0
Other income, net		0.0		0.0
Total (expenses) and other income		0.0		0.0
Income before income taxes		19.2		24.2
Benefit of income taxes		6.2		8.0
Income of consolidated companies		13.0		16.2
Equity in net income of unconsolidated affiliates		0.0		0.0
NET INCOME		13.0		16.2
Net (income) loss attributable to noncontrolling interests		0.0		0.0
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS		$ 13.0		$ 16.2
Earnings per share, Basic		$ 0.03		$ 0.04
Earnings per share, Diluted		$ 0.03		$ 0.04
Weighted-average number of common shares outstanding, Basic		0.0		0.0
Weighted-average number of common shares outstanding, Diluted		0.0		0.0