Principles of Consolidation and Other Matters	3 Months Ended
Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Principles of Consolidation and Other Matters	Principles of Consolidation and Other Matters The consolidated financial statements included herein have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission. While certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been omitted pursuant to such rules and regulations for interim filings, the Company believes that the information and disclosures presented are adequate to make such information and disclosures not misleading. These consolidated financial statements should be read in conjunction with the consolidated financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 (the "2018 Form 10-K"). The financial information contained herein reflects all normal recurring adjustments which are, in the opinion of management, necessary for a fair presentation of the Company’s consolidated financial statements as of and for the three month periods ended March 31, 2019 and 2018. Cash and Cash Equivalents Cash and cash equivalents primarily consist of certificates of deposit and time deposits, with original maturities of three months or less, and money market funds. The estimated fair value of the Company's cash and cash equivalents approximates their carrying value. The Company is required to maintain operating funds primarily related to regulatory requirements outside of the United States or as collateral under captive insurance arrangements. At March 31, 2019, the Company maintained $196 million related to these regulatory requirements. Funds Held in Escrow For Acquisition The Company received proceeds from debt issuances related to the JLT Transaction in the first quarter of 2019, which were placed in escrow. At March 31, 2019, these funds were reported as funds held in escrow for acquisition in the consolidated balance sheet. The funds were released from escrow upon the completion of the Transaction in April 2019. Investments The caption "Investment income (loss)" in the consolidated statements of income comprises realized and unrealized gains and losses from investments recognized in earnings. It includes, when applicable, other than temporary declines in the value of securities, mark-to-market increases or decreases in equity investments with readily determinable fair values and equity method gains or losses on the Company's investments in private equity funds. The Company holds investments in certain private equity funds that are accounted for under the equity method of accounting using a consistently applied three-month lag period adjusted for any known significant changes from the lag period to the reporting date of the Company. The underlying private equity funds follow investment company accounting, where investments within the fund are carried at fair value. Investment gains or losses for the Company's proportionate share of the change in fair value of the funds are recorded in earnings. Investments accounted for using the equity method of accounting are included in "other assets" in the consolidated balance sheets. The Company recorded net investment income of $5 million for the three months ended March 31, 2019 compared to a net investment loss of less than $1 million for the same period in the prior year. The three month period ending March 31, 2019 includes gains of $3 million related to mark-to-market changes in equity securities and gains of $2 million related to investments in private equity funds and other investments. Leases Effective January 1, 2019, the Company adopted the new accounting guidance related to leases. Under the new guidance, a lessee is required to recognize assets and liabilities for its leases with lease terms more than 12 months. The Company adopted this new standard using the modified retrospective method, which applies the new guidance beginning with the year of adoption, with the cumulative effect of initially applying the guidance recognized as an adjustment to retained earnings at January 1, 2019. There was no cumulative-effect adjustment required to be booked to retained earnings upon transition. Prior period results have not been restated to reflect the adoption of this new standard. On January 1, 2019, the Company recognized a lease liability of $1.9 billion and a corresponding right-of-use asset ("ROU asset") of $1.7 billion, including the reclassification of approximately $0.2 billion of unamortized lease incentives and restructuring liabilities, upon the adoption of this standard, with minimal impact on the consolidated statement of income. See Note 12 for further information related to Leases. Income Taxes The Company's effective tax rate in the first quarter of 2019 was 23% compared with 23.9% in the first quarter of 2018. The tax rates in both periods reflect the impact of discrete tax matters such as excess tax benefits related to share-based compensation, tax legislation, changes in uncertain tax positions, deferred tax adjustments and nontaxable adjustments to contingent acquisition consideration. The excess tax benefit related to share-based payments is the most significant discrete item, reducing the effective tax rate by 3.2% and 2.4% in the first quarters of 2019 and 2018, respectively. The Company is routinely examined by tax authorities in the jurisdictions in which it has significant operations. The Company regularly considers the likelihood of assessments in each of the taxing jurisdictions resulting from examinations. When evaluating the potential imposition of penalties, the Company considers a number of relevant factors under penalty statutes, including appropriate disclosure of the tax return position, the existence of legal authority supporting the Company's position, and reliance on the opinion of professional tax advisors. The Company reports a liability for unrecognized tax benefits resulting from uncertain tax positions taken or expected to be taken in tax returns. The Company's gross unrecognized tax benefits decreased from $78 million at December 31, 2018 to $74 million at March 31, 2019 due to current accruals offset by settlements of audits and expirations of statutes of limitation. It is reasonably possible that the total amount of unrecognized tax benefits will decrease between zero and approximately $10 million within the next twelve months due to settlements of audits and expirations of statutes of limitation.

Principles of Consolidation and Other Matters

The consolidated financial statements included herein have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission. While certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been omitted pursuant to such rules and regulations for interim filings, the Company believes that the information and disclosures presented are adequate to make such information and disclosures not misleading. These consolidated financial statements should be read in conjunction with the consolidated financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 (the "2018 Form 10-K").

The financial information contained herein reflects all normal recurring adjustments which are, in the opinion of management, necessary for a fair presentation of the Company’s consolidated financial statements as of and for the three month periods ended March 31, 2019 and 2018.

Cash and Cash Equivalents

Cash and cash equivalents primarily consist of certificates of deposit and time deposits, with original maturities of three months or less, and money market funds. The estimated fair value of the Company's cash and cash equivalents approximates their carrying value. The Company is required to maintain operating funds primarily related to regulatory requirements outside of the United States or as collateral under captive insurance arrangements. At March 31, 2019, the Company maintained $196 million related to these regulatory requirements.

Funds Held in Escrow For Acquisition

The Company received proceeds from debt issuances related to the JLT Transaction in the first quarter of 2019, which were placed in escrow. At March 31, 2019, these funds were reported as funds held in escrow for acquisition in the consolidated balance sheet. The funds were released from escrow upon the completion of the Transaction in April 2019.

Investments

The caption "Investment income (loss)" in the consolidated statements of income comprises realized and unrealized gains and losses from investments recognized in earnings. It includes, when applicable, other than temporary declines in the value of securities, mark-to-market increases or decreases in equity investments with readily determinable fair values and equity method gains or losses on the Company's investments in private equity funds.

The Company holds investments in certain private equity funds that are accounted for under the equity method of accounting using a consistently applied three-month lag period adjusted for any known significant changes from the lag period to the reporting date of the Company. The underlying private equity funds follow investment company accounting, where investments within the fund are carried at fair value. Investment gains or losses for the Company's proportionate share of the change in fair value of the funds are recorded in earnings. Investments accounted for using the equity method of accounting are included in "other assets" in the consolidated balance sheets.

The Company recorded net investment income of $5 million for the three months ended March 31, 2019 compared to a net investment loss of less than $1 million for the same period in the prior year. The three month period ending March 31, 2019 includes gains of $3 million related to mark-to-market changes in equity securities and gains of $2 million related to investments in private equity funds and other investments.

Leases

Effective January 1, 2019, the Company adopted the new accounting guidance related to leases. Under the new guidance, a lessee is required to recognize assets and liabilities for its leases with lease terms more than 12 months. The Company adopted this new standard using the modified retrospective method, which applies the new guidance beginning with the year of adoption, with the cumulative effect of initially applying the guidance recognized as an adjustment to retained earnings at January 1, 2019. There was no cumulative-effect adjustment required to be booked to retained earnings upon transition. Prior period results have not been restated to reflect the adoption of this new standard.

On January 1, 2019, the Company recognized a lease liability of $1.9 billion and a corresponding right-of-use asset ("ROU asset") of $1.7 billion, including the reclassification of approximately $0.2 billion of unamortized lease incentives and restructuring liabilities, upon the adoption of this standard, with minimal impact on the consolidated statement of income.

See Note 12 for further information related to Leases.

Income Taxes

The Company's effective tax rate in the first quarter of 2019 was 23% compared with 23.9% in the first quarter of 2018. The tax rates in both periods reflect the impact of discrete tax matters such as excess tax benefits related to share-based compensation, tax legislation, changes in uncertain tax positions, deferred tax adjustments and nontaxable adjustments to contingent acquisition consideration. The excess tax benefit related to share-based payments is the most significant discrete item, reducing the effective tax rate by 3.2% and 2.4% in the first quarters of 2019 and 2018, respectively.

The Company is routinely examined by tax authorities in the jurisdictions in which it has significant operations. The Company regularly considers the likelihood of assessments in each of the taxing jurisdictions resulting from examinations. When evaluating the potential imposition of penalties, the Company considers a number of relevant factors under penalty statutes, including appropriate disclosure of the tax return position, the existence of legal authority supporting the Company's position, and reliance on the opinion of professional tax advisors.

The Company reports a liability for unrecognized tax benefits resulting from uncertain tax positions taken or expected to be taken in tax returns. The Company's gross unrecognized tax benefits decreased from $78 million at December 31, 2018 to $74 million at March 31, 2019 due to current accruals offset by settlements of audits and expirations of statutes of limitation. It is reasonably possible that the total amount of unrecognized tax benefits will decrease between zero and approximately $10 million within the next twelve months due to settlements of audits and expirations of statutes of limitation.