Interest income earned on both U.S. and foreign fiduciary funds held in trust.

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Number of Business Groups

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No definition available.

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Net uncollected premiums and claims and the related payables are not assets and liabilities of MMC and are not included in the accompanying consolidated balance sheets.

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The carrying amount of assets an investment adviser manages on behalf of investors.

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No definition available.

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- Definition

For each balance sheet presented, the amount of accumulated amortization for capitalized computer software costs.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 985
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The carrying amount of capitalized computer software costs net of accumulated amortization as of the balance sheet date.

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- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

The aggregate amount of income from investments (for example, dividends) not considered a component of the entity's core operations.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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-Subparagraph (SX 210.5-03.7)
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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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No definition available.

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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No definition available.

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The amount of previously reported deferred or unearned revenue that was recognized as revenue during the period. For cash flows, this element primarily pertains to amortization of deferred credits on long-term arrangements. As a noncash item, it is deducted from net income when calculating cash provided by or used in operations using the indirect method.

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The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Number of options outstanding, including both vested and non-vested options.

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Summary of Significant Accounting Policies (Narratives) (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) business_group segment shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($) shares
Schedule of Equity Method Investments [Line Items]
Number of business segments \| segment	2
Uncollected Premiums and Claims, Net of Related Payables	$ 6,900,000,000	$ 7,300,000,000
Restricted cash and cash equivalents	209,000,000
Investment income	$ 38,000,000	37,000,000	$ 69,000,000
Intangible asset useful life	10 years
Indefinite-lived intangible assets	$ 0	0
Capitalized computer software costs	498,000,000	501,000,000
Capitalized computer software costs accumulated amortization	$ 958,000,000	$ 837,000,000
Stock options outstanding (in shares) \| shares	14,776,804	17,995,082	22,600,000
Trident III \| Private Equity Funds
Schedule of Equity Method Investments [Line Items]
Recognition of deferred performance fees	$ 29,000,000	$ 31,000,000
Trident II \| Private Equity Funds
Schedule of Equity Method Investments [Line Items]
Recognition of deferred performance fees			$ 41,000,000
Mercer Consulting Group
Schedule of Equity Method Investments [Line Items]
Assets in trusts or funds for management or trustee fee	$ 21,000,000,000
Minimum \| Software Development
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	3 years
Maximum \| Software Development
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	10 years
Risk and Insurance Services Segment
Schedule of Equity Method Investments [Line Items]
Interest income, fiduciary assets	$ 21,000,000	24,000,000	27,000,000
Consulting Segment
Schedule of Equity Method Investments [Line Items]
Number of business groups \| business_group	2
Interest income, fiduciary assets	$ 4,000,000	$ 6,000,000	5,000,000
Customer Lists \| Minimum
Schedule of Equity Method Investments [Line Items]
Intangible asset useful life	10 years
Customer Lists \| Maximum
Schedule of Equity Method Investments [Line Items]
Intangible asset useful life	15 years
Performance Fees \| Trident II
Schedule of Equity Method Investments [Line Items]
Recognition of deferred performance fees			$ 15,000,000