Presentation of Financial Statements	12 Months Ended
Dec. 31, 2017
Presentation of Financial Statements [Abstract]
Presentation of Financial Statements	Presentation of Financial Statements The terms “Omnicom,” “the Company,” “we,” “our” and “us” each refer to Omnicom Group Inc. and its subsidiaries, unless the context indicates otherwise. The accompanying consolidated financial statements were prepared in accordance with generally accepted accounting principles in the United States, or U.S. GAAP or GAAP. All intercompany balances and transactions have been eliminated. We prepare our financial statements in conformity with U.S. GAAP and are required to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates and assumptions. Accounting Changes On January 1, 2017, we adopted FASB ASU 2016-09, Compensation - Stock Compensation: Improvements to Employee Share-Based Payment Accounting, or ASU 2016-09, which requires that beginning in 2017 excess tax benefits and deficiencies related to share-based compensation be recorded in results of operations upon vesting of restricted stock awards or exercise of stock options. Excess tax benefits and deficiencies represent the difference between the actual compensation deduction for tax purposes, which is calculated as the difference between the grant date price of the award and the price of our common stock on the vesting or exercise date, and compensation expense recognized for financial reporting purposes. In prior years, excess tax benefits and deficiencies were recorded in additional paid-in capital. In 2017 we recognized an excess tax benefit of $20.8 million. ASU 2016-09 requires that cash flows related to the excess tax benefits or deficiencies be classified in operating activities. Accordingly, we retrospectively adjusted the statement of cash flows for 2016 and 2015 to conform to the current year presentation, resulting in an increase in net cash provided by operating activities and a corresponding decrease in net cash used in financing activities of $21.2 million and $27.2 million, respectively. Further, ASU 2016-09 permits a policy election to either continue to estimate the number of awards that will be forfeited or to account for forfeitures as they occur. We elected to account for forfeitures as they occur. Accordingly, we recorded a cumulative catch-up adjustment to increase additional paid-in capital and reduce opening retained earnings by $4.5 million reflecting the estimate of unvested awards at December 31, 2016 that were not expected to vest. On January 1, 2017, we adopted FASB ASU 2016-16, Income Taxes: Intra-Entity Transfers of Assets Other than Inventory, or ASU 2016-16, which requires that the income tax effects of intra-entity transfers of assets other than inventory are recognized when the transfer occurs. We adopted ASU 2016-16 using the modified retrospective method and recorded a cumulative catch-up adjustment to reduce opening retained earnings by $27.1 million reflecting the elimination of the deferred tax asset related to intercompany asset transfers. On December 31, 2017, we adopted FASB ASU 2017-12, Derivatives and Hedging, or ASU 2017-12, which amended the hedge accounting and recognition and presentation requirements. The adoption of ASU 2017-12 did not have any impact on our existing hedges, financial position or results of operations.

Presentation of Financial Statements

The terms “Omnicom,” “the Company,” “we,” “our” and “us” each refer to Omnicom Group Inc. and its subsidiaries, unless the context indicates otherwise. The accompanying consolidated financial statements were prepared in accordance with generally accepted accounting principles in the United States, or U.S. GAAP or GAAP. All intercompany balances and transactions have been eliminated.

We prepare our financial statements in conformity with U.S. GAAP and are required to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates and assumptions.

Accounting Changes

On January 1, 2017, we adopted FASB ASU 2016-09, Compensation - Stock Compensation: Improvements to Employee Share-Based Payment Accounting, or ASU 2016-09, which requires that beginning in 2017 excess tax benefits and deficiencies related to share-based compensation be recorded in results of operations upon vesting of restricted stock awards or exercise of stock options. Excess tax benefits and deficiencies represent the difference between the actual compensation deduction for tax purposes, which is calculated as the difference between the grant date price of the award and the price of our common stock on the vesting or exercise date, and compensation expense recognized for financial reporting purposes. In prior years, excess tax benefits and deficiencies were recorded in additional paid-in capital. In 2017 we recognized an excess tax benefit of $20.8 million.

ASU 2016-09 requires that cash flows related to the excess tax benefits or deficiencies be classified in operating activities. Accordingly, we retrospectively adjusted the statement of cash flows for 2016 and 2015 to conform to the current year presentation, resulting in an increase in net cash provided by operating activities and a corresponding decrease in net cash used in financing activities of $21.2 million and $27.2 million, respectively. Further, ASU 2016-09 permits a policy election to either continue to estimate the number of awards that will be forfeited or to account for forfeitures as they occur. We elected to account for forfeitures as they occur. Accordingly, we recorded a cumulative catch-up adjustment to increase additional paid-in capital and reduce opening retained earnings by $4.5 million reflecting the estimate of unvested awards at December 31, 2016 that were not expected to vest.

On January 1, 2017, we adopted FASB ASU 2016-16, Income Taxes: Intra-Entity Transfers of Assets Other than Inventory, or ASU 2016-16, which requires that the income tax effects of intra-entity transfers of assets other than inventory are recognized when the transfer occurs. We adopted ASU 2016-16 using the modified retrospective method and recorded a cumulative catch-up adjustment to reduce opening retained earnings by $27.1 million reflecting the elimination of the deferred tax asset related to intercompany asset transfers.

On December 31, 2017, we adopted FASB ASU 2017-12, Derivatives and Hedging, or ASU 2017-12, which amended the hedge accounting and recognition and presentation requirements. The adoption of ASU 2017-12 did not have any impact on our existing hedges, financial position or results of operations.