New Accounting Guidance (Policies)	6 Months Ended
Jun. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New Accounting Pronouncements	New Accounting Guidance In March 2017, the FASB issued new guidance that changes the presentation of net periodic pension cost and net periodic postretirement cost (''net periodic benefit costs"). The new guidance requires employers to report the service cost component of net periodic benefit costs in the same line item as other compensation costs in the income statement. The other components of net periodic benefit costs are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations, if one is presented. In addition, only the service cost component is eligible for capitalization, when applicable. The guidance is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. Early adoption is permitted as of the beginning of an annual period for which financial statements (interim or annual) have not been issued or made available for issuance. The new guidance requires retrospective application for the presentation of the service cost component and the other components of net periodic benefit costs, and prospective application for the capitalization of the service cost component. The adoption of this guidance will impact the presentation of the Company's results of operations, in particular, reducing net operating income, but will have no impact on the Company's net income. In January 2017, the FASB issued new guidance to simplify the test for goodwill impairment. The new guidance eliminates the second step in the current two-step goodwill impairment process, under which a goodwill impairment loss is measured by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill for that reporting unit. The new guidance requires a one-step impairment test, in which the goodwill impairment charge is based on the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. The guidance should be applied on a prospective basis with the nature of and reason for the change in accounting principle disclosed upon transition. The guidance is effective for annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company does not expect the adoption of this standard to have a material impact on the Company's financial position or results of operations. In January 2017, the FASB issued guidance which clarifies the definition of a business in order to assist companies with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. Early adoption is permitted under certain circumstances and the guidance must be applied prospectively as of the beginning of the period of adoption. The Company is currently evaluating the impact, if any, the adoption of this standard will have on its financial position or results of operations. In November 2016, the FASB issued new guidance which requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. As a result, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The new guidance is effective for public companies for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The guidance must be applied retrospectively to all periods presented. Early adoption is permitted. The adoption of this guidance is not expected to have an impact on the Company's consolidated balance sheets or consolidated statements of cash flows. In October 2016, the FASB also issued new guidance which requires an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The new guidance eliminates the exception for an intra-entity transfer of an asset other than inventory. The new guidance is effective for public companies for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The new guidance must be applied on a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the period of adoption. The Company is currently evaluating the impact the adoption of this standard will have on its financial position and results of operations. In August 2016, the FASB issued new guidance which adds or clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows, including cash payments for debt prepayments or debt extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees. The guidance is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The guidance must be applied retrospectively to all periods presented unless retrospective application is impracticable. Early adoption is permitted. The Company is currently evaluating the impact the guidance will have on its statement of cash flows. In February 2016, the FASB issued new guidance intended to improve financial reporting for leases. Under the new guidance, a lessee will be required to recognize assets and liabilities for leases with lease terms of more than 12 months. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee primarily will depend on its classification as a financing or operating lease. However, unlike current GAAP, which requires that only capital leases be recognized on the balance sheet, the new guidance requires that both types of leases be recognized on the balance sheet. The new guidance will require additional disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. These disclosures include qualitative and quantitative requirements, and additional information about the amounts recorded in the financial statements. The accounting by organizations that own the assets ("lessor") leased by the lessee will remain largely unchanged from current GAAP. However, the guidance contains some targeted improvements that are intended to align, where necessary, lessor accounting with the lessee accounting model and with the updated revenue recognition guidance issued in 2014. The new guidance on leases will take effect for public companies for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early application will be permitted. The Company is currently evaluating the impact the adoption of the guidance will have on its financial position and results of operations, but expects material "right to use" assets and lease liabilities to be recorded on its consolidated balance sheets. In January 2016, the FASB issued new guidance intended to improve the recognition and measurement of financial instruments. The new guidance requires equity investments (except those accounted for under the equity method of accounting, or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income; requires public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes; requires separate presentation of financial assets and financial liabilities by measurement category and form of financial asset (i.e., securities or loans and receivables) on the balance sheet or the accompanying notes to the financial statements; eliminates the requirement for public business entities to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet; and requires a reporting organization to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk (also referred to as "own credit") when the organization has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. The new guidance is effective for public companies for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The Company is currently evaluating the impact of the adoption of the guidance on its financial position and results of operations. New Revenue Recognition Pronouncement In May 2014, the FASB issued new accounting guidance to clarify the principles for revenue recognition. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of 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. To achieve that principle, the entity should apply the following steps: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. Entities are permitted to adopt the guidance under one of the following methods: the "full retrospective" method, which applies the guidance to each period presented (prior years restated) or the "modified retrospective" method in which the guidance is only applied to the year of adoption, with the cumulative effect of initially applying the guidance recognized as an adjustment to retained earnings. The Company will adopt the new guidance effective January 1, 2018. The Company is evaluating the transition method it will use, but currently expects to use the modified retrospective method. The Company continues to evaluate the impact of the new standard. Based on the results of our reviews to date, the Company expects there will be significant movement in the quarterly timing of revenue recognition in the Risk and Insurance Services segment. In particular, in the Company’s reinsurance broking operations recognition of revenue will be accelerated under the new standard. Currently, revenue related to certain reinsurance placements is recognized on the later of billing or effective date as premiums are written and attached to the reinsurance treaties. This typically results in revenue being recognized over a 12 to 24 month period. Under the new guidance, revenue will be recognized largely at the policy effective date, subject to the constraint as defined in the guidance. In the insurance brokerage operations, revenue from commission based arrangements will continue to be recorded at the policy effective date, while the timing of revenue recognition for certain fee based arrangements will shift among quarters. Since the vast majority of our fee arrangements involve contracts that cover a single year of services, the Company does not believe there will be a significant change to the amount of revenue recognized in an annual period once the standard is adopted. The Company is currently evaluating, designing and implementing changes in processes, controls and systems necessary to support the new revenue recognition requirements. The Company's initial review and preliminary conclusions for the Consulting segment indicate the application of the new standard will not materially change the existing timing of revenue recognition in quarterly or annual periods. The conclusions will be completed following a final review of customer arrangement legal terms and conditions in certain non-U.S. locations. The Company expects that certain costs that are currently expensed will be capitalized as costs to obtain or costs to fulfill customer contracts under guidance issued as part of the revenue recognition standard. Costs to obtain a contract, generally, would be amortized over the expected life of the underlying customer relationship. However, in many cases, costs to obtain a contract renewal are commensurate with the costs to obtain the initial contract. Those costs would have an amortization period of less than one year, and the Company would expect to use the practical expedient available in the standard and expense such costs as incurred. Any capitalized costs to fulfill a contract would be amortized on a basis consistent with the transfer of services to which they relate. Such expenses related to Risk and Insurance Services would generally be "amortized" over a period of one year or less. The Company is continuing to quantify the amount of such costs that must be deferred. In the Consulting segment, certain implementation expenses that are currently deferred may have a longer amortization period to reflect the amortization over the expected life of the contract including anticipated renewal periods. The Company has not yet fully quantified the impact of the changes discussed above. However, the Company expects that it will recognize a significant amount of contract assets and/or receivables upon implementation of the standard, related to both the contract assets for capitalized costs to obtain and fulfill contracts, as well as commissions receivable related to reinsurance brokerage activity. New Accounting Pronouncements Recently Adopted In October 2016, the FASB issued new guidance which changes the evaluation of whether a reporting entity is the primary beneficiary of a variable interest entity by changing how a reporting entity that is a single decision maker of a variable interest entity treats indirect interests in the entity held through related parties that are under common control with the reporting entity. If a reporting entity satisfies the first characteristic of a primary beneficiary (such that it is the single decision maker of a variable interest entity), the new guidance requires that reporting entity, in determining whether it satisfies the second characteristic of a primary beneficiary, to include all of its direct variable interest in a variable interest entity and, on a proportionate basis, its indirect variable interests in a variable interest entity held through related parties, including related parties that are under common control with the reporting entity. The adoption of this guidance did not have a significant impact on its financial position, results of operations and statement of cash flows. In April 2016, the FASB issued new guidance which simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures and statutory tax withholding requirements, as well as classification in the statement of cash flows. The new guidance requires that companies record all excess tax benefits and tax deficiencies as an income tax benefit or expense in the income statement and classify excess tax benefits as an operating activity in the statement of cash flows. The Company adopted this new guidance prospectively, effective January 1, 2017 and prior periods have not been adjusted. For the six months ended June 30, 2017, the adoption of this new standard reduced income tax expense in the consolidated statement of income by approximately $48 million. For the six months ended June 30, 2016, the Company recorded an excess tax benefit of $24 million as an increase to equity in its consolidated balance sheet, which was reflected as cash provided by financing activities in the consolidated statement of cash flows. In March 2016, the FASB issued new guidance which eliminates the requirement that when an investment qualifies for use of the equity method as a result of an increase in the level of ownership interest or degree of influence, an investor must adjust the investment, results of operations and retained earnings retroactively on a step-by-step basis as if the equity method had been in effect during all previous periods that the investment had been held. The amendments require that the equity method investor add the cost of acquiring the additional interest in the investee to the current basis of the investor’s previously held interest and adopt the equity method of accounting as of the date the investment becomes qualified for equity method accounting. Therefore, upon qualifying for the equity method of accounting, no retroactive adjustment of the investment is required. The amendments require that an entity that has an available-for-sale equity security that becomes qualified for the equity method of accounting recognize through earnings the unrealized holding gain or loss in accumulated other comprehensive income at the date the investment becomes qualified for use of the equity method. The new guidance is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. The guidance was adopted on January 1, 2017 and did not have an impact on the Company's financial position or results of operations.

New Accounting Guidance

In March 2017, the FASB issued new guidance that changes the presentation of net periodic pension cost and net periodic postretirement cost (''net periodic benefit costs"). The new guidance requires employers to report the service cost component of net periodic benefit costs in the same line item as other compensation costs in the income statement. The other components of net periodic benefit costs are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations, if one is presented. In addition, only the service cost component is eligible for capitalization, when applicable. The guidance is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. Early adoption is permitted as of the beginning of an annual period for which financial statements (interim or annual) have not been issued or made available for issuance. The new guidance requires retrospective application for the presentation of the service cost component and the other components of net periodic benefit costs, and prospective application for the capitalization of the service cost component. The adoption of this guidance will impact the presentation of the Company's results of operations, in particular, reducing net operating income, but will have no impact on the Company's net income.

In January 2017, the FASB issued new guidance to simplify the test for goodwill impairment. The new guidance eliminates the second step in the current two-step goodwill impairment process, under which a goodwill impairment loss is measured by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill for that reporting unit. The new guidance requires a one-step impairment test, in which the goodwill impairment charge is based on the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. The guidance should be applied on a prospective basis with the nature of and reason for the change in accounting principle disclosed upon transition. The guidance is effective for annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted. The Company does not expect the adoption of this standard to have a material impact on the Company's financial position or results of operations.

In January 2017, the FASB issued guidance which clarifies the definition of a business in order to assist companies with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. Early adoption is permitted under certain circumstances and the guidance must be applied prospectively as of the beginning of the period of adoption. The Company is currently evaluating the impact, if any, the adoption of this standard will have on its financial position or results of operations.

In November 2016, the FASB issued new guidance which requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. As a result, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The new guidance is effective for public companies for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The guidance must be applied retrospectively to all periods presented. Early adoption is permitted. The adoption of this guidance is not expected to have an impact on the Company's consolidated balance sheets or consolidated statements of cash flows.

In October 2016, the FASB also issued new guidance which requires an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The new guidance eliminates the exception for an intra-entity transfer of an asset other than inventory. The new guidance is effective for public companies for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The new guidance must be applied on a modified retrospective basis through a cumulative-effect adjustment to retained earnings as of the beginning of the period of adoption. The Company is currently evaluating the impact the adoption of this standard will have on its financial position and results of operations.

In August 2016, the FASB issued new guidance which adds or clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows, including cash payments for debt prepayments or debt extinguishment costs, contingent consideration payments made after a business combination and distributions received from equity method investees. The guidance is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The guidance must be applied retrospectively to all periods presented unless retrospective application is impracticable. Early adoption is permitted. The Company is currently evaluating the impact the guidance will have on its statement of cash flows.

In February 2016, the FASB issued new guidance intended to improve financial reporting for leases. Under the new guidance, a lessee will be required to recognize assets and liabilities for leases with lease terms of more than 12 months. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee primarily will depend on its classification as a financing or operating lease. However, unlike current GAAP, which requires that only capital leases be recognized on the balance sheet, the new guidance requires that both types of leases be recognized on the balance sheet. The new guidance will require additional disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. These disclosures include qualitative and quantitative requirements, and additional information about the amounts recorded in the financial statements. The accounting by organizations that own the assets ("lessor") leased by the lessee will remain largely unchanged from current GAAP. However, the guidance contains some targeted improvements that are intended to align, where necessary, lessor accounting with the lessee accounting model and with the updated revenue recognition guidance issued in 2014. The new guidance on leases will take effect for public companies for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early application will be permitted. The Company is currently evaluating the impact the adoption of the guidance will have on its financial position and results of operations, but expects material "right to use" assets and lease liabilities to be recorded on its consolidated balance sheets.

In January 2016, the FASB issued new guidance intended to improve the recognition and measurement of financial instruments. The new guidance requires equity investments (except those accounted for under the equity method of accounting, or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income; requires public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes; requires separate presentation of financial assets and financial liabilities by measurement category and form of financial asset (i.e., securities or loans and receivables) on the balance sheet or the accompanying notes to the financial statements; eliminates the requirement for public business entities to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet; and requires a reporting organization to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk (also referred to as "own credit") when the organization has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. The new guidance is effective for public companies for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The Company is currently evaluating the impact of the adoption of the guidance on its financial position and results of operations.

New Revenue Recognition Pronouncement

In May 2014, the FASB issued new accounting guidance to clarify the principles for revenue recognition. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of 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. To achieve that principle, the entity should apply the following steps: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. Entities are permitted to adopt the guidance under one of the following methods: the "full retrospective" method, which applies the guidance to each period presented (prior years restated) or the "modified retrospective" method in which the guidance is only applied to the year of adoption, with the cumulative effect of initially applying the guidance recognized as an adjustment to retained earnings. The Company will adopt the new guidance effective January 1, 2018. The Company is evaluating the transition method it will use, but currently expects to use the modified retrospective method.

The Company continues to evaluate the impact of the new standard. Based on the results of our reviews to date, the Company expects there will be significant movement in the quarterly timing of revenue recognition in the Risk and Insurance Services segment. In particular, in the Company’s reinsurance broking operations recognition of revenue will be accelerated under the new standard. Currently, revenue related to certain reinsurance placements is recognized on the later of billing or effective date as premiums are written and attached to the reinsurance treaties. This typically results in revenue being recognized over a 12 to 24 month period. Under the new guidance, revenue will be recognized largely at the policy effective date, subject to the constraint as defined in the guidance. In the insurance brokerage operations, revenue from commission based arrangements will continue to be recorded at the policy effective date, while the timing of revenue recognition for certain fee based arrangements will shift among quarters. Since the vast majority of our fee arrangements involve contracts that cover a single year of services, the Company does not believe there will be a significant change to the amount of revenue recognized in an annual period once the standard is adopted. The Company is currently evaluating, designing and implementing changes in processes, controls and systems necessary to support the new revenue recognition requirements.

The Company's initial review and preliminary conclusions for the Consulting segment indicate the application of the new standard will not materially change the existing timing of revenue recognition in quarterly or annual periods. The conclusions will be completed following a final review of customer arrangement legal terms and conditions in certain non-U.S. locations.

The Company expects that certain costs that are currently expensed will be capitalized as costs to obtain or costs to fulfill customer contracts under guidance issued as part of the revenue recognition standard. Costs to obtain a contract, generally, would be amortized over the expected life of the underlying customer relationship. However, in many cases, costs to obtain a contract renewal are commensurate with the costs to obtain the initial contract. Those costs would have an amortization period of less than one year, and the Company would expect to use the practical expedient available in the standard and expense such costs as incurred. Any capitalized costs to fulfill a contract would be amortized on a basis consistent with the transfer of services to which they relate. Such expenses related to Risk and Insurance Services would generally be "amortized" over a period of one year or less. The Company is continuing to quantify the amount of such costs that must be deferred. In the Consulting segment, certain implementation expenses that are currently deferred may have a longer amortization period to reflect the amortization over the expected life of the contract including anticipated renewal periods.

The Company has not yet fully quantified the impact of the changes discussed above. However, the Company expects that it will recognize a significant amount of contract assets and/or receivables upon implementation of the standard, related to both the contract assets for capitalized costs to obtain and fulfill contracts, as well as commissions receivable related to reinsurance brokerage activity.

New Accounting Pronouncements Recently Adopted

In October 2016, the FASB issued new guidance which changes the evaluation of whether a reporting entity is the primary beneficiary of a variable interest entity by changing how a reporting entity that is a single decision maker of a variable interest entity treats indirect interests in the entity held through related parties that are under common control with the reporting entity. If a reporting entity satisfies the first characteristic of a primary beneficiary (such that it is the single decision maker of a variable interest entity), the new guidance requires that reporting entity, in determining whether it satisfies the second characteristic of a primary beneficiary, to include all of its direct variable interest in a variable interest entity and, on a proportionate basis, its indirect variable interests in a variable interest entity held through related parties, including related parties that are under common control with the reporting entity. The adoption of this guidance did not have a significant impact on its financial position, results of operations and statement of cash flows.

In April 2016, the FASB issued new guidance which simplifies several aspects of the accounting for employee share-based payment transactions, including the accounting for income taxes, forfeitures and statutory tax withholding requirements, as well as classification in the statement of cash flows. The new guidance requires that companies record all excess tax benefits and tax deficiencies as an income tax benefit or expense in the income statement and classify excess tax benefits as an operating activity in the statement of cash flows. The Company adopted this new guidance prospectively, effective January 1, 2017 and prior periods have not been adjusted. For the six months ended June 30, 2017, the adoption of this new standard reduced income tax expense in the consolidated statement of income by approximately $48 million. For the six months ended June 30, 2016, the Company recorded an excess tax benefit of $24 million as an increase to equity in its consolidated balance sheet, which was reflected as cash provided by financing activities in the consolidated statement of cash flows.

In March 2016, the FASB issued new guidance which eliminates the requirement that when an investment qualifies for use of the equity method as a result of an increase in the level of ownership interest or degree of influence, an investor must adjust the investment, results of operations and retained earnings retroactively on a step-by-step basis as if the equity method had been in effect during all previous periods that the investment had been held. The amendments require that the equity method investor add the cost of acquiring the additional interest in the investee to the current basis of the investor’s previously held interest and adopt the equity method of accounting as of the date the investment becomes qualified for equity method accounting. Therefore, upon qualifying for the equity method of accounting, no retroactive adjustment of the investment is required. The amendments require that an entity that has an available-for-sale equity security that becomes qualified for the equity method of accounting recognize through earnings the unrealized holding gain or loss in accumulated other comprehensive income at the date the investment becomes qualified for use of the equity method. The new guidance is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. The guidance was adopted on January 1, 2017 and did not have an impact on the Company's financial position or results of operations.