Accounting Principles and Practices	6 Months Ended
Jun. 30, 2016
Accounting Policies [Abstract]
Accounting Principles and Practices	Accounting Principles and Practices New Accounting Pronouncements Credit Losses In June 2016, the FASB issued new accounting guidance on the measurement of credit losses on financial instruments. The new guidance 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. An entity will apply the new guidance through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is effective. The guidance is effective for Aon in the first quarter of 2020 and early adoption is permitted beginning in the first quarter of 2019. Aon is currently evaluating the impact that the standard will have on its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption. Share-based Compensation In March 2016, the FASB issued new accounting guidance on several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The new guidance requires all excess tax benefits and tax deficiencies to be recognized as income tax expense or benefit in the income statement and treated as discrete items in the reporting period. Further, excess tax benefits are required to be classified along with other income tax cash flows as an operating activity. Amendments related to the timing of when excess tax benefits are recognized, minimum statutory withholding requirements, forfeitures, and intrinsic value should be applied using a modified retrospective transition method by means of a cumulative-effect adjustment to equity as of the beginning of the period in which the guidance is adopted. Amendments related to the presentation of employee taxes paid on the statement of cash flows when an employer withholds shares to meet the minimum statutory withholding requirement should be applied retrospectively. Amendments requiring recognition of excess tax benefits and tax deficiencies in the income statement and the practical expedient for estimating expected term should be applied prospectively. An entity may elect to apply the amendments related to the presentation of excess tax benefits on the statement of cash flows using either a prospective transition method or a retrospective transition method. The guidance is effective for Aon in the first quarter of 2017 and early adoption is permitted. Aon is currently evaluating the impact that the standard will have on its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption. Leases In February 2016, the FASB issued new accounting guidance on leases, which requires lessees to recognize assets and liabilities for most leases. Under the new guidance, a lessee should recognize in the statement of financial position a liability to make lease payments and a right-of-use asset representing its right to use the underlying asset for the lease term. The recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee have not significantly changed from currently effective U.S. GAAP. The new standard will be effective for the Company in the first quarter of 2019, with early application permitted. In transition, lessees and lessors are required to recognize and measure leases at the beginning of the earliest period presented using a modified retrospective approach. The modified retrospective approach includes a number of optional practical expedients that entities may elect to apply. These practical expedients relate to the identification and classification of leases that commenced before the effective date, initial direct costs for leases that commenced before the effective date, and the ability to use hindsight in evaluating lessee options to extend or terminate a lease or to purchase the underlying asset. Aon is currently evaluating the impact the standard will have on its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption. Financial Assets and Liabilities In January 2016, the FASB issued new accounting guidance on recognition and measurement of financial assets and financial liabilities. The amendments in the new guidance make targeted improvements, which include the requirement to measure equity investments with readily determinable fair values at fair value through net income, simplification of the impairment assessment for equity investments without readily determinable fair values, adjustments to existing and additional disclosure requirements, and additional tax considerations. An entity should apply the amendments by means of a cumulative-effect adjustment to the balance sheet as of the beginning of the fiscal year of adoption. The amendments related to equity securities without readily determinable fair values (including disclosure requirements) should be applied prospectively to equity investments that exist as of the date of adoption of the guidance. The guidance is effective for the Company in the first quarter of 2018 and early adoption is permitted. Aon is currently evaluating the impact that the standard will have on the its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption. Presentation of Deferred Taxes In November 2015, the FASB issued new accounting guidance on the balance sheet presentation of deferred taxes, which requires that deferred tax liabilities and assets be classified as non-current. Aon early adopted this guidance in the second quarter of 2016 and retrospectively applied its requirements to all periods presented. For the year ended December 31, 2015, Aon reclassified its current deferred tax positions to non-current and netted the new balances by jurisdiction, which increased Non-current deferred tax assets by $92 million and decreased Non-current deferred tax liabilities by $139 million on the Condensed Consolidated Statement of Financial Position. Debt Issuance Costs In April 2015, the 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 will also be applied to Aon's debt issuance costs related to its line-of-credit arrangements. This guidance was 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. Consolidations In February 2015, the FASB issued new accounting guidance on consolidations, which will eliminate the deferral granted to investment companies from applying the variable interest entities guidance and make targeted amendments to the current consolidation guidance. The new guidance applies to all entities involved with limited partnerships or similar entities and requires re-evaluation of these entities under the revised guidance, which could change previous consolidation conclusions. The guidance was effective for the Company in the first quarter of 2016. The adoption of this guidance did not have a material impact on the Company's Condensed Consolidated Financial Statements. Revenue Recognition In May 2014, the FASB issued new accounting guidance on revenue from contracts with customers, which, when effective, will supersede nearly all existing revenue recognition guidance under U.S. GAAP. The core principal of the guidance is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The guidance also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. The guidance is effective for the Company in the first quarter of 2018 and early adoption is permitted beginning the first quarter of 2017. The guidance permits two methods of transition upon adoption: full retrospective and modified retrospective. Under the full retrospective method, prior periods would be restated under the new revenue standard, providing a comparable view across all periods presented. Under the modified retrospective method, prior periods would not be restated. Rather, revenues and other disclosures for pre-2018 periods would be provided in the notes to the financial statements as previously reported under the current revenue standard. Aon is currently evaluating the impact that the standard will have on the Company's Condensed Consolidated Financial Statements. The Company is also determining the appropriate method of transition to the guidance, but expects to adopt upon the effective date of January 1, 2018.

Accounting Principles and Practices

New Accounting Pronouncements

Credit Losses

In June 2016, the FASB issued new accounting guidance on the measurement of credit losses on financial instruments. The new guidance 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. An entity will apply the new guidance through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is effective. The guidance is effective for Aon in the first quarter of 2020 and early adoption is permitted beginning in the first quarter of 2019. Aon is currently evaluating the impact that the standard will have on its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption.

Share-based Compensation

In March 2016, the FASB issued new accounting guidance on several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The new guidance requires all excess tax benefits and tax deficiencies to be recognized as income tax expense or benefit in the income statement and treated as discrete items in the reporting period. Further, excess tax benefits are required to be classified along with other income tax cash flows as an operating activity. Amendments related to the timing of when excess tax benefits are recognized, minimum statutory withholding requirements, forfeitures, and intrinsic value should be applied using a modified retrospective transition method by means of a cumulative-effect adjustment to equity as of the beginning of the period in which the guidance is adopted. Amendments related to the presentation of employee taxes paid on the statement of cash flows when an employer withholds shares to meet the minimum statutory withholding requirement should be applied retrospectively. Amendments requiring recognition of excess tax benefits and tax deficiencies in the income statement and the practical expedient for estimating expected term should be applied prospectively. An entity may elect to apply the amendments related to the presentation of excess tax benefits on the statement of cash flows using either a prospective transition method or a retrospective transition method. The guidance is effective for Aon in the first quarter of 2017 and early adoption is permitted. Aon is currently evaluating the impact that the standard will have on its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption.

Leases

In February 2016, the FASB issued new accounting guidance on leases, which requires lessees to recognize assets and liabilities for most leases. Under the new guidance, a lessee should recognize in the statement of financial position a liability to make lease payments and a right-of-use asset representing its right to use the underlying asset for the lease term. The recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee have not significantly changed from currently effective U.S. GAAP. The new standard will be effective for the Company in the first quarter of 2019, with early application permitted. In transition, lessees and lessors are required to recognize and measure leases at the beginning of the earliest period presented using a modified retrospective approach. The modified retrospective approach includes a number of optional practical expedients that entities may elect to apply. These practical expedients relate to the identification and classification of leases that commenced before the effective date, initial direct costs for leases that commenced before the effective date, and the ability to use hindsight in evaluating lessee options to extend or terminate a lease or to purchase the underlying asset. Aon is currently evaluating the impact the standard will have on its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption.

Financial Assets and Liabilities

In January 2016, the FASB issued new accounting guidance on recognition and measurement of financial assets and financial liabilities. The amendments in the new guidance make targeted improvements, which include the requirement to measure equity investments with readily determinable fair values at fair value through net income, simplification of the impairment assessment for equity investments without readily determinable fair values, adjustments to existing and additional disclosure requirements, and additional tax considerations. An entity should apply the amendments by means of a cumulative-effect adjustment to the balance sheet as of the beginning of the fiscal year of adoption. The amendments related to equity securities without readily determinable fair values (including disclosure requirements) should be applied prospectively to equity investments that exist as of the date of adoption of the guidance. The guidance is effective for the Company in the first quarter of 2018 and early adoption is permitted. Aon is currently evaluating the impact that the standard will have on the its Condensed Consolidated Financial Statements, as well as the method of transition and period of adoption.

Presentation of Deferred Taxes

In November 2015, the FASB issued new accounting guidance on the balance sheet presentation of deferred taxes, which requires that deferred tax liabilities and assets be classified as non-current. Aon early adopted this guidance in the second quarter of 2016 and retrospectively applied its requirements to all periods presented. For the year ended December 31, 2015, Aon reclassified its current deferred tax positions to non-current and netted the new balances by jurisdiction, which increased Non-current deferred tax assets by $92 million and decreased Non-current deferred tax liabilities by $139 million on the Condensed Consolidated Statement of Financial Position.

Debt Issuance Costs

In April 2015, the 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 will also be applied to Aon's debt issuance costs related to its line-of-credit arrangements. This guidance was 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.

Consolidations

In February 2015, the FASB issued new accounting guidance on consolidations, which will eliminate the deferral granted to investment companies from applying the variable interest entities guidance and make targeted amendments to the current consolidation guidance. The new guidance applies to all entities involved with limited partnerships or similar entities and requires re-evaluation of these entities under the revised guidance, which could change previous consolidation conclusions. The guidance was effective for the Company in the first quarter of 2016. The adoption of this guidance did not have a material impact on the Company's Condensed Consolidated Financial Statements.

Revenue Recognition

In May 2014, the FASB issued new accounting guidance on revenue from contracts with customers, which, when effective, will supersede nearly all existing revenue recognition guidance under U.S. GAAP. The core principal of the guidance is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The guidance also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. The guidance is effective for the Company in the first quarter of 2018 and early adoption is permitted beginning the first quarter of 2017. The guidance permits two methods of transition upon adoption: full retrospective and modified retrospective. Under the full retrospective method, prior periods would be restated under the new revenue standard, providing a comparable view across all periods presented. Under the modified retrospective method, prior periods would not be restated. Rather, revenues and other disclosures for pre-2018 periods would be provided in the notes to the financial statements as previously reported under the current revenue standard. Aon is currently evaluating the impact that the standard will have on the Company's Condensed Consolidated Financial Statements. The Company is also determining the appropriate method of transition to the guidance, but expects to adopt upon the effective date of January 1, 2018.