Accounting Principles and Practices (Policies)	9 Months Ended
Sep. 30, 2020
Accounting Policies [Abstract]
Basis of Presentation	The accompanying unaudited Condensed Consolidated Financial Statements and Notes thereto (the “Financial Statements”) have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”). The Financial Statements include the accounts of Aon plc, an Irish public limited company, and all of its controlled subsidiaries. Intercompany accounts and transactions have been eliminated. The Financial Statements include, in the opinion of management, all adjustments (consisting of normal recurring adjustments and reclassifications) necessary to present fairly the Company’s consolidated financial position, results of operations, and cash flows for all periods presented.Certain information and disclosures normally included in the Financial Statements prepared in accordance with U.S. GAAP have been condensed or omitted.
Use of Estimates	Use of Estimates The preparation of the Financial Statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosures of contingent assets and liabilities at the date of the Financial Statements, and the reported amounts of reserves and expenses. These estimates and assumptions are based on management’s best estimates and judgments. Management evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors, including the current economic environment. Management believes its estimates to be reasonable given the current facts available. Aon adjusts such estimates and assumptions when facts and circumstances dictate. Illiquid credit markets, volatile equity markets, foreign currency exchange rate movements, and, recently, impacts from the COVID-19 pandemic increase the uncertainty inherent in such estimates and assumptions. As future events and their effects cannot be determined with precision, actual results could differ significantly from these estimates. Changes in estimates resulting from continuing changes in the economic environment would, if applicable, be reflected in the Financial Statements in future periods.
New Accounting Standards	Adoption of New Accounting Standards Cloud Computing Arrangements In August 2018, the Financial Accounting Standards Board (“FASB”) issued new accounting guidance on implementation costs incurred in a cloud computing arrangement that is a service contract. The new guidance aligns capitalization requirements for certain implementation costs incurred in cloud computing arrangements with existing requirements for capitalizing implementation costs for internal-use software. These costs will be deferred over the term of the hosting arrangement, including any optional renewal periods the entity is reasonably certain to exercise. An entity may apply the new guidance on either a prospective or retrospective basis. The new guidance was effective for Aon in the first quarter of 2020 and was adopted on a prospective basis for all implementation costs incurred after the date of initial adoption. The adoption of this guidance had no significant impact on the Financial Statements. Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued new accounting guidance on simplifying the test for goodwill impairment. Currently the standard requires an entity to perform a two-step test to determine the amount, if any, of goodwill impairment. In Step 1, an entity compares the fair value of a reporting unit with its carrying amount, including goodwill. If the carrying amount of the reporting unit exceeds its fair value, the entity performs Step 2 and compares the implied fair value of goodwill with the carrying amount of that goodwill for that reporting unit. An impairment charge equal to the amount by which the carrying amount of goodwill for the reporting unit exceeds the implied fair value of that goodwill is recorded, limited to the amount of goodwill allocated to that reporting unit. The new guidance removes Step 2. An entity will apply a one-step quantitative test and record the amount of goodwill impairment as the excess of a reporting unit’s carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The new guidance does not amend the optional qualitative assessment of goodwill impairment. The new guidance was effective for Aon in the first quarter of 2020 and was adopted on a prospective basis. The adoption of this guidance had no impact on the Financial Statements. Credit Losses In June 2016, the FASB issued a new accounting standard on the measurement of credit losses on financial instruments. The new standard replaces the current incurred loss impairment methodology with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The Company adopted the new standard as of January 1, 2020 using the modified retrospective approach. Under this approach, prior periods were not restated. Rather, the cumulative effect of initially applying the new standard was recognized as an adjustment to retained earnings. Upon the adoption of this guidance on January 1, 2020, the Company recognized a cumulative adjustment of $6 million to decrease retained earnings. The Company’s estimate for allowance for credit losses with respect to receivables is based on a combination of factors, including evaluation of forward-looking information, historical write-offs, aging of balances, and other qualitative and quantitative analyses. Financial Disclosures about Guarantors In March 2020, the SEC issued a final rule that amended the disclosure requirements related to certain registered securities under SEC Regulation S-X Rules 3-10 and 3-16 of Regulation S-X. The changes are intended to provide investors with material information given the specific facts and circumstances, make disclosures easier to understand, and reduce the cost and burdens to registrants. The final rule replaces the requirement to provide condensed consolidated financial information with a requirement to present summarized financial information of the issuers and guarantors in the registrant’s Management Discussion and Analysis section or in the financial statements and reduces the periods for which summarized financial information is presented to the most recent annual period and year-to-date interim period. The amendments are effective January 4, 2021, with early adoption permitted. The Company elected to early adopt the final rule in the second quarter of 2020 and elected to present these disclosures in Item 2, Management’s Discussion and Analysis of Financial Condition and Results of Operations. Accounting Standards Issued But Not Yet Adopted Simplifying the Accounting for Income Taxes In December 2019, the FASB issued new accounting guidance that simplifies the accounting for income taxes by removing certain exceptions to the general principles in the existing guidance. It also clarifies certain aspects of the existing guidance to promote more consistent application. The new guidance is effective for Aon in the first quarter of 2021, with early adoption permitted. The Company previously expected to adopt this guidance in the first quarter of 2021 and is now considering early adoption in the fourth quarter of 2020. Different components of the guidance require retrospective, modified retrospective, or prospective adoption. The Company does not expect there to be a significant impact on the Financial Statements. Changes to the Disclosure Requirements for Defined Benefit Plans In August 2018, the FASB issued new accounting guidance related to the disclosure requirements for employers that sponsor defined benefit pension and other postretirement benefit plans. The guidance requires sponsors of these plans to provide additional disclosures, including weighted average interest rates used in the entity’s cash balance pension plans and a narrative description of reasons for any significant gains or losses impacting the benefit obligation for the period, and eliminates certain previous disclosure requirements. The new guidance is effective for Aon in the first quarter of 2021 and will be applied retrospectively in the Notes to Condensed Consolidated Financial Statements.
Derivatives	The Company is exposed to market risks, including changes in foreign currency exchange rates and interest rates. To manage the risk related to these exposures, the Company enters into various derivative instruments that reduce these risks by creating offsetting exposures. The Company does not enter into derivative transactions for trading or speculative purposes. Foreign Exchange Risk Management The Company is exposed to foreign exchange risk when it earns revenues, pays expenses, enters into monetary intercompany transfers or other transactions denominated in a currency that differs from its functional currency. The Company uses foreign exchange derivatives, typically forward contracts, options and cross currency swaps, to reduce its overall exposure to the effects of currency fluctuations on cash flows. These exposures are hedged, on average, for less than two years. These derivatives are accounted for as hedges, and changes in fair value are recorded each period in Other comprehensive income (loss) in the Condensed Consolidated Statements of Comprehensive Income. The Company also uses foreign exchange derivatives, typically forward contracts and options, to economically hedge the currency exposure of the Company’s global liquidity profile, including monetary assets or liabilities that are denominated in a non-functional currency of an entity, typically on a rolling 30-day basis, but may be for up to one year in the future. These derivatives are not accounted for as hedges, and changes in fair value are recorded each period in Other income (expense) in the Condensed Consolidated Statements of Income.
Fair Value Measurement	The following methods and assumptions are used to estimate the fair values of the Company’s financial instruments: Money market funds consist of institutional prime, treasury, and government money market funds. The Company reviews treasury and government money market funds to obtain reasonable assurance that the fund net asset value is $1 per share, and reviews the floating net asset value of institutional prime money market funds for reasonableness. Equity investments consist of equity securities and equity derivatives valued using the closing stock price on a national securities exchange. Over the counter equity derivatives are valued using observable inputs such as underlying prices of the underlying security and volatility. On a sample basis, the Company reviews the listing of Level 1 equity securities in the portfolio, agrees the closing stock prices to a national securities exchange, and independently verifies the observable inputs for Level 2 equity derivatives and securities. Fixed income investments consist of certain categories of bonds and derivatives. Corporate, government, and agency bonds are valued by pricing vendors who estimate fair value using recently executed transactions and proprietary models based on observable inputs, such as interest rate spreads, yield curves, and credit risk. Asset-backed securities are valued by pricing vendors who estimate fair value using discounted cash flow models utilizing observable inputs based on trade and quote activity of securities with similar features. Fixed income derivatives are valued by pricing vendors using observable inputs such as interest rates and yield curves. The Company obtains an understanding of the models, inputs, and assumptions used in developing prices provided by its vendors through discussions with the fund managers. The Company independently verifies the observable inputs, as well as assesses assumptions used for reasonableness based on relevant market conditions and internal Company guidelines. If an assumption is deemed unreasonable, based on internal Company guidelines, it is then reviewed by management and the fair value estimate provided by the vendor is adjusted, if deemed appropriate. These adjustments do not occur frequently and historically are not material to the fair value estimates used in the Financial Statements. Derivatives are carried at fair value, based upon industry standard valuation techniques that use, where possible, current market-based or independently sourced pricing inputs, such as interest rates, currency exchange rates, or implied volatility. Debt is carried at outstanding principal balance, less any unamortized issuance costs, discount or premium. Fair value is based on quoted market prices or estimates using discounted cash flow analyses based on current borrowing rates for similar types of borrowing arrangements.

Accounting Principles and Practices (Policies)

9 Months Ended

Sep. 30, 2020

The accompanying unaudited Condensed Consolidated Financial Statements and Notes thereto (the “Financial Statements”) have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”). The Financial Statements include the accounts of Aon plc, an Irish public limited company, and all of its controlled subsidiaries. Intercompany accounts and transactions have been eliminated. The Financial Statements include, in the opinion of management, all adjustments (consisting of normal recurring adjustments and reclassifications) necessary to present fairly the Company’s consolidated financial position, results of operations, and cash flows for all periods presented.Certain information and disclosures normally included in the Financial Statements prepared in accordance with U.S. GAAP have been condensed or omitted.

Use of Estimates

The preparation of the Financial Statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosures of contingent assets and liabilities at the date of the Financial Statements, and the reported amounts of reserves and expenses. These estimates and assumptions are based on management’s best estimates and judgments. Management evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors, including the current economic environment. Management believes its estimates to be reasonable given the current facts available. Aon adjusts such estimates and assumptions when facts and circumstances dictate. Illiquid credit markets, volatile equity markets, foreign currency exchange rate movements, and, recently, impacts from the COVID-19 pandemic increase the uncertainty inherent in such estimates and assumptions. As future events and their effects cannot be determined with precision, actual results could differ significantly from these estimates. Changes in estimates resulting from continuing changes in the economic environment would, if applicable, be reflected in the Financial Statements in future periods.

New Accounting Standards

Adoption of New Accounting Standards

Cloud Computing Arrangements

In August 2018, the Financial Accounting Standards Board (“FASB”) issued new accounting guidance on implementation costs incurred in a cloud computing arrangement that is a service contract. The new guidance aligns capitalization requirements for certain implementation costs incurred in cloud computing arrangements with existing requirements for capitalizing implementation costs for internal-use software. These costs will be deferred over the term of the hosting arrangement, including any optional renewal periods the entity is reasonably certain to exercise. An entity may apply the new guidance on either a prospective or retrospective basis. The new guidance was effective for Aon in the first quarter of 2020 and was adopted on a prospective basis for all implementation costs incurred after the date of initial adoption. The adoption of this guidance had no significant impact on the Financial Statements.

Simplifying the Test for Goodwill Impairment

In January 2017, the FASB issued new accounting guidance on simplifying the test for goodwill impairment. Currently the standard requires an entity to perform a two-step test to determine the amount, if any, of goodwill impairment. In Step 1, an entity compares the fair value of a reporting unit with its carrying amount, including goodwill. If the carrying amount of the reporting unit exceeds its fair value, the entity performs Step 2 and compares the implied fair value of goodwill with the carrying amount of that goodwill for that reporting unit. An impairment charge equal to the amount by which the carrying

amount of goodwill for the reporting unit exceeds the implied fair value of that goodwill is recorded, limited to the amount of goodwill allocated to that reporting unit. The new guidance removes Step 2. An entity will apply a one-step quantitative test and record the amount of goodwill impairment as the excess of a reporting unit’s carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The new guidance does not amend the optional qualitative assessment of goodwill impairment. The new guidance was effective for Aon in the first quarter of 2020 and was adopted on a prospective basis. The adoption of this guidance had no impact on the Financial Statements.

Credit Losses

In June 2016, the FASB issued a new accounting standard on the measurement of credit losses on financial instruments. The new standard replaces the current incurred loss impairment methodology with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The Company adopted the new standard as of January 1, 2020 using the modified retrospective approach. Under this approach, prior periods were not restated. Rather, the cumulative effect of initially applying the new standard was recognized as an adjustment to retained earnings. Upon the adoption of this guidance on January 1, 2020, the Company recognized a cumulative adjustment of $6 million to decrease retained earnings.

The Company’s estimate for allowance for credit losses with respect to receivables is based on a combination of factors, including evaluation of forward-looking information, historical write-offs, aging of balances, and other qualitative and quantitative analyses.

Financial Disclosures about Guarantors

In March 2020, the SEC issued a final rule that amended the disclosure requirements related to certain registered securities under SEC Regulation S-X Rules 3-10 and 3-16 of Regulation S-X. The changes are intended to provide investors with material information given the specific facts and circumstances, make disclosures easier to understand, and reduce the cost and burdens to registrants. The final rule replaces the requirement to provide condensed consolidated financial information with a requirement to present summarized financial information of the issuers and guarantors in the registrant’s Management Discussion and Analysis section or in the financial statements and reduces the periods for which summarized financial information is presented to the most recent annual period and year-to-date interim period. The amendments are effective January 4, 2021, with early adoption permitted.

The Company elected to early adopt the final rule in the second quarter of 2020 and elected to present these disclosures in Item 2, Management’s Discussion and Analysis of Financial Condition and Results of Operations.

Accounting Standards Issued But Not Yet Adopted

Simplifying the Accounting for Income Taxes

In December 2019, the FASB issued new accounting guidance that simplifies the accounting for income taxes by removing certain exceptions to the general principles in the existing guidance. It also clarifies certain aspects of the existing guidance to promote more consistent application. The new guidance is effective for Aon in the first quarter of 2021, with early adoption permitted. The Company previously expected to adopt this guidance in the first quarter of 2021 and is now considering early adoption in the fourth quarter of 2020. Different components of the guidance require retrospective, modified retrospective, or prospective adoption. The Company does not expect there to be a significant impact on the Financial Statements.

Changes to the Disclosure Requirements for Defined Benefit Plans

In August 2018, the FASB issued new accounting guidance related to the disclosure requirements for employers that sponsor defined benefit pension and other postretirement benefit plans. The guidance requires sponsors of these plans to provide additional disclosures, including weighted average interest rates used in the entity’s cash balance pension plans and a narrative description of reasons for any significant gains or losses impacting the benefit obligation for the period, and eliminates certain previous disclosure requirements. The new guidance is effective for Aon in the first quarter of 2021 and will be applied retrospectively in the Notes to Condensed Consolidated Financial Statements.

Derivatives

The Company is exposed to market risks, including changes in foreign currency exchange rates and interest rates. To manage the risk related to these exposures, the Company enters into various derivative instruments that reduce these risks by creating offsetting exposures. The Company does not enter into derivative transactions for trading or speculative purposes.

Foreign Exchange Risk Management

The Company is exposed to foreign exchange risk when it earns revenues, pays expenses, enters into monetary intercompany transfers or other transactions denominated in a currency that differs from its functional currency. The Company uses foreign exchange derivatives, typically forward contracts, options and cross currency swaps, to reduce its overall exposure to the effects of currency fluctuations on cash flows. These exposures are hedged, on average, for less than two years. These derivatives are accounted for as hedges, and changes in fair value are recorded each period in Other comprehensive income (loss) in the Condensed Consolidated Statements of Comprehensive Income.

The Company also uses foreign exchange derivatives, typically forward contracts and options, to economically hedge the currency exposure of the Company’s global liquidity profile, including monetary assets or liabilities that are denominated in a non-functional currency of an entity, typically on a rolling 30-day basis, but may be for up to one year in the future. These derivatives are not accounted for as hedges, and changes in fair value are recorded each period in Other income (expense) in the Condensed Consolidated Statements of Income.

Fair Value Measurement

The following methods and assumptions are used to estimate the fair values of the Company’s financial instruments:

Money market funds consist of institutional prime, treasury, and government money market funds. The Company reviews treasury and government money market funds to obtain reasonable assurance that the fund net asset value is $1 per share, and reviews the floating net asset value of institutional prime money market funds for reasonableness.

Equity investments consist of equity securities and equity derivatives valued using the closing stock price on a national securities exchange. Over the counter equity derivatives are valued using observable inputs such as underlying prices of the underlying security and volatility. On a sample basis, the Company reviews the listing of Level 1 equity securities in the portfolio, agrees the closing stock prices to a national securities exchange, and independently verifies the observable inputs for Level 2 equity derivatives and securities.

Fixed income investments consist of certain categories of bonds and derivatives. Corporate, government, and agency bonds are valued by pricing vendors who estimate fair value using recently executed transactions and proprietary models based on observable inputs, such as interest rate spreads, yield curves, and credit risk. Asset-backed securities are valued by pricing vendors who estimate fair value using discounted cash flow models utilizing observable inputs based on trade and quote activity of securities with similar features. Fixed income derivatives are valued by pricing vendors using observable inputs such as interest rates and yield curves. The Company obtains an understanding of the models, inputs, and assumptions used in developing prices provided by its vendors through discussions with the fund managers. The Company independently verifies the observable inputs, as well as assesses assumptions used for reasonableness based on relevant market conditions and internal Company guidelines. If an assumption is deemed unreasonable, based on internal Company guidelines, it is then reviewed by management and the fair value estimate provided by the vendor is adjusted, if deemed appropriate. These adjustments do not occur frequently and historically are not material to the fair value estimates used in the Financial Statements.

Derivatives are carried at fair value, based upon industry standard valuation techniques that use, where possible, current market-based or independently sourced pricing inputs, such as interest rates, currency exchange rates, or implied volatility.

Debt is carried at outstanding principal balance, less any unamortized issuance costs, discount or premium. Fair value is based on quoted market prices or estimates using discounted cash flow analyses based on current borrowing rates for similar types of borrowing arrangements.