Accounting Principles and Practices	3 Months Ended
Mar. 31, 2016
Accounting Policies [Abstract]
Accounting Principles and Practices	Accounting Principles and Practices New Accounting Pronouncements 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. The impact from the adoption of this guidance on the Company's Condensed Consolidated Financial Statements cannot be determined at this time. 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. 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. The guidance is effective for Aon in the first quarter of 2017, however, the Company expects to early adopt this guidance in 2016 and retrospectively apply its requirements to all periods presented. The adoption of this guidance is not expected to have a material impact on the Company's Condensed Consolidated Financial Statements. 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. The guidance is effective for the Company in the first quarter of 2018 and early adoption is permitted. The Company is currently evaluating the impact that the standard will have on the Company's Condensed Consolidated Financial Statements. 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. The Company is currently evaluating the impact the standard will have on the Company's 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. 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 the Company's Condensed Consolidated Financial Statements.

Accounting Principles and Practices

New Accounting Pronouncements

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. The impact from the adoption of this guidance on the Company's Condensed Consolidated Financial Statements cannot be determined at this time. 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.

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. The guidance is effective for Aon in the first quarter of 2017, however, the Company expects to early adopt this guidance in 2016 and retrospectively apply its requirements to all periods presented. The adoption of this guidance is not expected to have a material impact on the Company's Condensed Consolidated Financial Statements.

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. The guidance is effective for the Company in the first quarter of 2018 and early adoption is permitted. The Company is currently evaluating the impact that the standard will have on the Company's Condensed Consolidated Financial Statements.

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. The Company is currently evaluating the impact the standard will have on the Company's 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. 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 the Company's Condensed Consolidated Financial Statements.