Revenue Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents	$ 1,860.2	$ 790.9
Accounts Receivable, net of allowance of $42.7	4,009.1	4,585.0
Expenditures billable to clients		0.0
Accounts receivable, billable to clients	1,995.9	1,747.4
Assets held for sale	5.7	5.7
Other current assets	459.6	346.5
Total current assets	8,330.5	7,475.5
Property and equipment, net of accumulated depreciation of $1,036.2	602.9	650.4
Deferred income taxes	270.2	234.0
Goodwill	3,784.7	3,820.4
Other non-current assets	524.8	524.4
TOTAL ASSETS	13,513.1	12,704.7
Accounts payable	5,515.1	6,420.2
Accrued liabilities	571.0	674.7
Contract liabilities	514.4	484.7
Short-term borrowings	82.6	84.9
Current portion of long-term debt	0.1	2.0
Liabilities held for sale	8.5	8.8
Total current liabilities	6,691.7	7,675.3
Long-term debt	3,261.4	1,285.6
Deferred compensation	438.5	476.6
Other non-current liabilities	791.6	768.8
TOTAL LIABILITIES	11,183.2	10,206.3
Redeemable noncontrolling interests	159.7	252.1	$ 238.0	$ 252.8
STOCKHOLDERS' EQUITY:
Common stock	39.1	38.6
Additional paid-in capital	1,041.8	955.2
Retained earnings	2,158.4	2,104.5
Accumulated other comprehensive loss, net of tax	(925.3)	(827.8)	(832.4)	(964.4)
Stockholders Equity Subtotal Before Treasury Stock	2,314.0	2,270.5
Less: Treasury stock	(176.1)	(59.0)
Total IPG stockholders' equity	2,137.9	2,211.5
Noncontrolling interests	32.3	34.8
TOTAL STOCKHOLDERS' EQUITY	2,170.2	2,246.3	2,115.8	2,090.4
TOTAL LIABILITIES AND EQUITY	$ 13,513.1	12,704.7
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents		790.9
Accounts Receivable, net of allowance of $42.7		4,585.0
Expenditures billable to clients		1,747.4
Accounts receivable, billable to clients		0.0
Assets held for sale		5.7
Other current assets		335.1
Total current assets		7,464.1
Property and equipment, net of accumulated depreciation of $1,036.2		650.4
Deferred income taxes		236.0
Goodwill		3,820.4
Other non-current assets		524.3
TOTAL ASSETS		12,695.2
Accounts payable		6,907.8
Accrued liabilities		674.7
Contract liabilities		0.0
Short-term borrowings		84.9
Current portion of long-term debt		2.0
Liabilities held for sale		8.8
Total current liabilities		7,678.2
Long-term debt		1,285.6
Deferred compensation		476.6
Other non-current liabilities		766.9
TOTAL LIABILITIES		10,207.3
Redeemable noncontrolling interests		252.1
STOCKHOLDERS' EQUITY:
Common stock		38.6
Additional paid-in capital		955.2
Retained earnings		2,093.6
Accumulated other comprehensive loss, net of tax		(827.4)
Stockholders Equity Subtotal Before Treasury Stock		2,260.0
Less: Treasury stock		(59.0)
Total IPG stockholders' equity		2,201.0
Noncontrolling interests		34.8
TOTAL STOCKHOLDERS' EQUITY		2,235.8
TOTAL LIABILITIES AND EQUITY		12,695.2
ASC 606 Adjustments | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cash and cash equivalents		0.0
Accounts Receivable, net of allowance of $42.7		0.0
Expenditures billable to clients		(1,747.4)
Accounts receivable, billable to clients		1,747.4
Assets held for sale		0.0
Other current assets		11.4
Total current assets		11.4
Property and equipment, net of accumulated depreciation of $1,036.2		0.0
Deferred income taxes		(2.0)
Goodwill		0.0
Other non-current assets		0.1
TOTAL ASSETS		9.5
Accounts payable		(487.6)
Accrued liabilities		0.0
Contract liabilities		484.7
Short-term borrowings		0.0
Current portion of long-term debt		0.0
Liabilities held for sale		0.0
Total current liabilities		(2.9)
Long-term debt		0.0
Deferred compensation		0.0
Other non-current liabilities		1.9
TOTAL LIABILITIES		(1.0)
Redeemable noncontrolling interests		0.0
STOCKHOLDERS' EQUITY:
Common stock		0.0
Additional paid-in capital		0.0
Retained earnings		10.9	75.3	35.6
Accumulated other comprehensive loss, net of tax		(0.4)	$ 0.3	$ (1.9)
Stockholders Equity Subtotal Before Treasury Stock		10.5
Less: Treasury stock		0.0
Total IPG stockholders' equity		10.5
Noncontrolling interests		0.0
TOTAL STOCKHOLDERS' EQUITY		10.5
TOTAL LIABILITIES AND EQUITY		$ 9.5