Pension and Other Postemployment Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plans [Abstract]
Defined contribution plans, contribution expense	$ 112.9	$ 108.5	$ 105.7
Defined Benefit Pension Plans [Member]
Components of Net Periodic Benefit Expense
Service cost	10.1	7.8	5.3
Interest cost	7.9	7.8	7.6
Expected return on plan assets	(3.6)	(3.7)	(4.0)
Amortization of prior service cost	4.6	4.5	4.3
Amortization of actuarial losses	7.0	5.3	5.7
Net periodic benefit expense	26.0	21.7	$ 18.9
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	90.0	98.0
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	56.0	$ 60.0
Unrecognized actuarial gains and losses expected to be amortized in the next year	$ 11.4
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Expense [Abstract]
Discount rate	3.50%	3.70%	3.50%
Compensation increases	2.00%	2.00%	1.90%
Expected return on plan assets	5.30%	4.80%	5.70%
Defined benefit pension plans, contributions by employer	$ 8.3	$ 6.6	$ 4.2
Benefit Obligation [Rollforward]
January 1	251.1	234.8
Service cost	10.1	7.8	5.3
Interest cost	7.9	7.8	7.6
Amendments, curtailments and settlements	0.3	0.0
Actuarial losses	6.8	13.3
Benefits paid	(9.1)	(9.4)
Foreign currency translation	9.9	(3.2)
December 31	277.0	251.1	234.8
Fair Value of Plan Assets [Rollforward]
January 1	68.6	68.9
Actual return on plan assets	6.3	4.9
Employer contributions	8.3	6.6	4.2
Benefits paid	(9.1)	(9.4)
Foreign currency translation and other	6.2	(2.4)
December 31	80.3	68.6	68.9
Funded Status [Abstract]
Funded Status December 31	(196.7)	(182.5)
Amounts Recorded in Balance Sheet, as Classified [Abstract]
Other assets	6.0	4.2
Other current liabilities	(5.1)	(5.1)
Long-term liabilities	(197.6)	(181.6)
Amounts Recorded in Balance Sheet	(196.7)	(182.5)
Accumulated benefit obligation	264.5	240.8
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Benefit obligation	253.8	241.3
Plan assets	51.1	54.6
Plans with benefit obligations in excess of plan assets, liabilities	$ 202.7	$ 186.7
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	3.10%	3.50%
Compensation increases	2.00%	2.00%
Estimated Future Benefit Payments [Abstract]
2018	$ 9.7
2019	10.8
2020	12.3
2021	16.1
2022	16.8
2023 - 2027	$ 99.6
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	100.00%
Percentage of Plan Assets, Actual Allocation	100.00%	100.00%
Defined Benefit Pension Plans [Member] | Level 1
Fair Value of Plan Assets [Rollforward]
January 1	$ 64.1
December 31	70.7	$ 64.1
Defined Benefit Pension Plans [Member] | Level 2
Fair Value of Plan Assets [Rollforward]
January 1	0.2
December 31	1.0	0.2
Defined Benefit Pension Plans [Member] | Level 3
Fair Value of Plan Assets [Rollforward]
January 1	4.3	4.2
Actual return on plan assets	0.2	0.2
Purchases, sales and settlements, net	4.1	(0.1)
December 31	8.6	4.3	4.2
Defined Benefit Pension Plans [Member] | Cash
Fair Value of Plan Assets [Rollforward]
January 1	1.6
December 31	$ 1.0	$ 1.6
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	4.00%
Percentage of Plan Assets, Actual Allocation	1.00%	3.00%
Defined Benefit Pension Plans [Member] | Cash | Level 1
Fair Value of Plan Assets [Rollforward]
January 1	$ 1.6
December 31	1.0	$ 1.6
Defined Benefit Pension Plans [Member] | Mutual funds
Fair Value of Plan Assets [Rollforward]
January 1	39.0
December 31	$ 41.5	$ 39.0
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	48.00%
Percentage of Plan Assets, Actual Allocation	52.00%	57.00%
Defined Benefit Pension Plans [Member] | Mutual funds | Level 1
Fair Value of Plan Assets [Rollforward]
January 1	$ 39.0
December 31	41.5	$ 39.0
Defined Benefit Pension Plans [Member] | Unit trusts
Fair Value of Plan Assets [Rollforward]
January 1	23.5
December 31	$ 28.2	$ 23.5
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	34.00%
Percentage of Plan Assets, Actual Allocation	35.00%	34.00%
Defined Benefit Pension Plans [Member] | Unit trusts | Level 1
Fair Value of Plan Assets [Rollforward]
January 1	$ 23.5
December 31	28.2	$ 23.5
Defined Benefit Pension Plans [Member] | Insurance contracts
Fair Value of Plan Assets [Rollforward]
January 1	4.3
December 31	$ 8.6	$ 4.3
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	11.00%
Percentage of Plan Assets, Actual Allocation	11.00%	6.00%
Defined Benefit Pension Plans [Member] | Insurance contracts | Level 3
Fair Value of Plan Assets [Rollforward]
January 1	$ 4.3
December 31	8.6	$ 4.3
Defined Benefit Pension Plans [Member] | Other
Fair Value of Plan Assets [Rollforward]
January 1	0.2
December 31	$ 1.0	$ 0.2
Weighted Average Asset Allocations [Abstract]
Percentage of Plan Assets, Target Allocation	3.00%
Percentage of Plan Assets, Actual Allocation	1.00%	0.00%
Defined Benefit Pension Plans [Member] | Other | Level 2
Fair Value of Plan Assets [Rollforward]
January 1	$ 0.2
December 31	$ 1.0	$ 0.2
Postemployment Arrangements [Member]
Defined Benefit Pension Plans and Postemployment Arrangements [Abstract]
Defined benefit plans, plan information	We have executive retirement agreements under which benefits will be paid to participants or to their beneficiaries over periods up to ten years beginning after cessation of full-time employment. Our postemployment arrangements are unfunded and benefits are paid when due.
Components of Net Periodic Benefit Expense
Service cost	$ 4.4	3.9	4.8
Interest cost	3.7	3.5	4.3
Amortization of prior service cost	3.5	3.1	3.2
Amortization of actuarial losses	1.0	1.1	1.6
Net periodic benefit expense	12.6	11.6	$ 13.9
Amounts Recognized in Accumulated Other Comprehensive Income [Abstract]
Unrecognized actuarial gains and losses and unrecognized prior service cost	54.0	51.0
Unrecognized actuarial gains and losses and unrecognized prior service cost, net of tax	33.0	$ 30.0
Unrecognized actuarial gains and losses expected to be amortized in the next year	$ 5.2
Weighted Average Assumptions Used to Determine the Net Periodic Benefit Expense [Abstract]
Discount rate	3.90%	4.10%	3.80%
Compensation increases	3.50%	3.50%	3.50%
Benefit Obligation [Rollforward]
January 1	$ 120.3	$ 115.9
Service cost	4.4	3.9	$ 4.8
Interest cost	3.7	3.5	4.3
Amendments, curtailments and settlements	0.0	5.6
Actuarial losses	8.1	0.6
Benefits paid	(8.8)	(9.2)
December 31	127.7	120.3	$ 115.9
Amounts Recorded in Balance Sheet, as Classified [Abstract]
Other current liabilities	(9.3)	(8.1)
Long-term liabilities	(118.4)	(112.2)
Amounts Recorded in Balance Sheet	$ (127.7)	$ (120.3)
Weighted Average Assumptions Used to Determine Benefit Obligation [Abstract]
Discount rate	3.40%	3.90%
Compensation increases	3.50%	3.50%
Estimated Future Benefit Payments [Abstract]
2018	$ 9.3
2019	7.4
2020	6.6
2021	6.1
2022	6.5
2023 - 2027	$ 33.2
Retention Plan [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Pension Plans and Postemployment Arrangements [Abstract]
Defined benefit plans, plan information	We have a Senior Executive Restrictive Covenant and Retention Plan, or Retention Plan, for certain executive officers selected by the Compensation Committee. The Retention Plan is a non-qualified deferred compensation severance plan that was adopted to secure non-competition, non-solicitation, non-disparagement and ongoing consulting services from such executive officers and to strengthen the retention aspect of executive officer compensation. The Retention Plan provides annual payments upon termination following at least seven years of service with Omnicom or its subsidiaries to the participants or to their beneficiaries. A participant’s annual benefit is payable for 15 consecutive calendar years following termination, but in no event prior to age 55. The annual benefit is equal to the lesser of (i) the participant’s final average pay times an applicable percentage, which is based upon the executive’s years of service as an executive officer, not to exceed 35% or (ii) $1.5 million adjusted for cost-of-living, beginning with the second annual payment, not to exceed 2.5% per year. The Retention Plan is not funded and benefits are paid when due.