Basis of Presentation	3 Months Ended
Mar. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited Condensed Consolidated Financial Statements and Notes thereto have been prepared in accordance with U.S. generally accepted accounting principles ("U.S. GAAP"). The Condensed Consolidated Financial Statements include the accounts of Aon plc and all of its controlled subsidiaries ("Aon" or the "Company"). All intercompany accounts and transactions have been eliminated. The Condensed Consolidated 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. These Condensed Consolidated Financial Statements should be read in conjunction with the Consolidated Financial Statements and Notes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2015. The results for the three months ended March 31, 2016 are not necessarily indicative of operating results that may be expected for the full year ending December 31, 2016. Reclassification Certain amounts in prior years' Condensed Consolidated Financial Statements and related notes have been reclassified to conform to the 2016 presentation. In prior periods, prepaid pensions were included in Other non-current assets in the Condensed Consolidated Statement of Financial Position. These amounts are now separately disclosed in the Condensed Consolidated Statement of Financial Position. Prepaid pensions were $737 million at March 31, 2016 and $1,033 million at December 31, 2015. Upon vesting of certain share-based payment arrangements, employees may elect to use a portion of the shares to satisfy tax withholding requirements, in which case Aon makes a payment to the taxing authority on the employee’s behalf and remits the remaining shares to the employee. The Company has historically presented amounts due to taxing authorities within Cash Flows From Operating Activities in the Condensed Consolidated Statements of Cash Flows. The amounts are now included in “Issuance of shares for employee benefit plans” within Cash Flows From Financing Activities. The Company believes this presentation provides greater clarity into the operating and financing activities of the Company as the substance and accounting for these transactions is that of a share repurchase. It also aligns the Company’s presentation to be consistent with industry practice and share-based compensation guidance issued by the Financial Accounting Standards Board ("FASB") in March 2016. Amounts reported in Issuance of shares for employee benefit plans were $128 million and $162 million, respectively, for the three months ended March 31, 2016 and March 31, 2015. These amounts, which were reclassified from Accounts payable and accrued liabilities and Other assets and liabilities, were $118 million and $44 million for the three months ended March 31, 2015. Changes to the presentation in the Condensed Consolidated Statements of Cash Flows for the three months ended March 31, 2015 were made related to certain line items within financing activities. In 2016, Purchases of shares from noncontrolling interests and Dividends paid to noncontrolling interests have been aggregated in a new line item titled “Noncontrolling interests and other financing activities” within financing activities. The balances held in these line items for the three months ended March 31, 2015 was $(5) million and $(1) million, respectively. In April 2015, the Financial Accounting Standards Board ("FASB") issued new accounting guidance on the presentation of debt issuance costs, which requires debt issuance costs to be presented in the balance sheet as a direct deduction from the associated debt liability. This guidance became effective for Aon in the first quarter of 2016, which required retrospective application to prior year comparable periods. For the year ended December 31, 2015, Aon reclassified $4 million from Other current assets and $33 million from Other non-current assets to Long-term debt on the Condensed Consolidated Statement of Financial Position. In the first quarter, Aon retrospectively adopted FASB's new accounting guidance on consolidations. No material changes were identified upon adoption of this new guidance. Use of Estimates The preparation of the accompanying Condensed Consolidated Financial Statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, 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, and foreign currency exchange rate movements increase the uncertainty inherent in such estimates and assumptions. As future events and their effects cannot be determined, among other factors, 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.

Basis of Presentation

The accompanying unaudited Condensed Consolidated Financial Statements and Notes thereto have been prepared in accordance with U.S. generally accepted accounting principles ("U.S. GAAP"). The Condensed Consolidated Financial Statements include the accounts of Aon plc and all of its controlled subsidiaries ("Aon" or the "Company"). All intercompany accounts and transactions have been eliminated. The Condensed Consolidated 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. These Condensed Consolidated Financial Statements should be read in conjunction with the Consolidated Financial Statements and Notes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2015. The results for the three months ended March 31, 2016 are not necessarily indicative of operating results that may be expected for the full year ending December 31, 2016.

Reclassification

Certain amounts in prior years' Condensed Consolidated Financial Statements and related notes have been reclassified to conform to the 2016 presentation.

In prior periods, prepaid pensions were included in Other non-current assets in the Condensed Consolidated Statement of Financial Position. These amounts are now separately disclosed in the Condensed Consolidated Statement of Financial Position. Prepaid pensions were $737 million at March 31, 2016 and $1,033 million at December 31, 2015.

Upon vesting of certain share-based payment arrangements, employees may elect to use a portion of the shares to satisfy tax withholding requirements, in which case Aon makes a payment to the taxing authority on the employee’s behalf and remits the remaining shares to the employee. The Company has historically presented amounts due to taxing authorities within Cash Flows From Operating Activities in the Condensed Consolidated Statements of Cash Flows. The amounts are now included in “Issuance of shares for employee benefit plans” within Cash Flows From Financing Activities. The Company believes this presentation provides greater clarity into the operating and financing activities of the Company as the substance and accounting for these transactions is that of a share repurchase. It also aligns the Company’s presentation to be consistent with industry practice and share-based compensation guidance issued by the Financial Accounting Standards Board ("FASB") in March 2016. Amounts reported in Issuance of shares for employee benefit plans were $128 million and $162 million, respectively, for the three months ended March 31, 2016 and March 31, 2015. These amounts, which were reclassified from Accounts payable and accrued liabilities and Other assets and liabilities, were $118 million and $44 million for the three months ended March 31, 2015.

Changes to the presentation in the Condensed Consolidated Statements of Cash Flows for the three months ended March 31, 2015 were made related to certain line items within financing activities. In 2016, Purchases of shares from noncontrolling interests and Dividends paid to noncontrolling interests have been aggregated in a new line item titled “Noncontrolling interests and other financing activities” within financing activities. The balances held in these line items for the three months ended March 31, 2015 was $(5) million and $(1) million, respectively.

In April 2015, the Financial Accounting Standards Board ("FASB") issued new accounting guidance on the presentation of debt issuance costs, which requires debt issuance costs to be presented in the balance sheet as a direct deduction from the associated debt liability. This guidance became effective for Aon in the first quarter of 2016, which required retrospective application to prior year comparable periods. For the year ended December 31, 2015, Aon reclassified $4 million from Other current assets and $33 million from Other non-current assets to Long-term debt on the Condensed Consolidated Statement of Financial Position.

In the first quarter, Aon retrospectively adopted FASB's new accounting guidance on consolidations. No material changes were identified upon adoption of this new guidance.

Use of Estimates

The preparation of the accompanying Condensed Consolidated Financial Statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, 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, and foreign currency exchange rate movements increase the uncertainty inherent in such estimates and assumptions. As future events and their effects cannot be determined, among other factors, 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.