Revenue Income Statement (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue	$ 1,895.7	$ 1,832.5	$ 5,617.9	$ 5,342.4
Billable expenses	401.8	375.7	1,240.5	1,115.4
Total revenue	2,297.5	2,208.2	6,858.4	6,457.8
OPERATING EXPENSES:
Salaries and related expenses	1,251.4	1,218.8	3,874.6	3,699.4
Office and other direct expenses	317.0	302.9	974.1	934.0
Billable expenses	401.8	375.7	1,240.5	1,115.4
Cost of services	1,970.2	1,897.4	6,089.2	5,748.8
Selling, general and administrative expenses	21.6	13.6	85.5	69.1
Depreciation and amortization	44.0	42.2	134.0	124.5
Total operating expenses	2,035.8	1,953.2	6,308.7	5,942.4
OPERATING INCOME	261.7	255.0	549.7	515.4
EXPENSES AND OTHER INCOME:
Interest expense	(27.6)	(21.0)	(73.6)	(67.6)
Interest income	5.3	4.1	14.0	14.0
Other expense, net	(15.3)	(9.9)	(56.0)	(24.5)
Total (expenses) and other income	(37.6)	(26.8)	(115.6)	(78.1)
Income before income taxes	224.1	228.2	434.1	437.3
Provision of income taxes	60.7	54.9	137.0	136.2
Income of consolidated companies	163.4	173.3	297.1	301.1
Equity in net income of unconsolidated affiliates	0.1	(1.0)	(1.9)	0.1
NET INCOME	163.5	172.3	295.2	301.2
Net (income) loss attributable to noncontrolling interests	(2.5)	(2.6)	(2.5)	0.9
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS	$ 161.0	$ 169.7	$ 292.7	$ 302.1
Earnings per share, Basic	$ 0.42	$ 0.44	$ 0.76	$ 0.77
Earnings per share, Diluted	$ 0.41	$ 0.43	$ 0.75	$ 0.76
Weighted-average number of common shares outstanding, Basic	382.6	389.5	383.2	391.2
Weighted-average number of common shares outstanding, Diluted	388.4	397.2	388.4	398.6
Dividends declared per common share	$ 0.21	$ 0.18	$ 0.63	$ 0.54
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue		$ 1,796.6		$ 5,282.3
Billable expenses		106.0		259.1
Total revenue		1,902.6		5,541.4
OPERATING EXPENSES:
Salaries and related expenses		1,218.8		3,699.4
Office and other direct expenses		302.9		934.0
Billable expenses		106.0		259.1
Cost of services		1,627.7		4,892.5
Selling, general and administrative expenses		13.6		69.1
Depreciation and amortization		42.2		124.5
Total operating expenses		1,683.5		5,086.1
OPERATING INCOME		219.1		455.3
EXPENSES AND OTHER INCOME:
Interest expense		(21.0)		(67.6)
Interest income		4.1		14.0
Other expense, net		(9.9)		(24.5)
Total (expenses) and other income		(26.8)		(78.1)
Income before income taxes		192.3		377.2
Provision of income taxes		42.5		115.8
Income of consolidated companies		149.8		261.4
Equity in net income of unconsolidated affiliates		(1.0)		0.1
NET INCOME		148.8		261.5
Net (income) loss attributable to noncontrolling interests		(2.6)		0.9
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS		$ 146.2		$ 262.4
Earnings per share, Basic		$ 0.38		$ 0.67
Earnings per share, Diluted		$ 0.37		$ 0.66
Weighted-average number of common shares outstanding, Basic		389.5		391.2
Weighted-average number of common shares outstanding, Diluted		397.2		398.6
Dividends declared per common share		$ 0.18		$ 0.54
ASC 606 Adjustments | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenue		$ 35.9		$ 60.1
Billable expenses		269.7		856.3
Total revenue		305.6		916.4
OPERATING EXPENSES:
Salaries and related expenses		0.0		0.0
Office and other direct expenses		0.0		0.0
Cost of services		269.7		856.3
Selling, general and administrative expenses		0.0		0.0
Depreciation and amortization		0.0		0.0
Total operating expenses		269.7		856.3
OPERATING INCOME		35.9		60.1
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		35.9		60.1
Provision of income taxes		12.4		20.4
Income of consolidated companies		23.5		39.7
Equity in net income of unconsolidated affiliates		0.0		0.0
NET INCOME		23.5		39.7
Net (income) loss attributable to noncontrolling interests		0.0		0.0
NET INCOME AVAILABLE TO IPG COMMON STOCKHOLDERS		$ 23.5		$ 39.7
Earnings per share, Basic		$ 0.06		$ 0.10
Earnings per share, Diluted		$ 0.06		$ 0.10
Weighted-average number of common shares outstanding, Basic		0.0		0.0
Weighted-average number of common shares outstanding, Diluted		0.0		0.0