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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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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Percentage of voting equity interests acquired at the acquisition date in the business combination.

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-Publisher FASB

-Name Accounting Standards Codification

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

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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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Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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

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

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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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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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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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Summary of Significant Accounting Policies (Narratives) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 segment	Dec. 31, 2013	Dec. 31, 2012	Feb. 24, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Indefinite-lived intangible assets	$ 0	$ 0
Schedule of Equity Method Investments [Line Items]
Number of business segments	2us-gaap_NumberOfOperatingSegments
Uncollected Premiums and Claims, Net of Related Payables	7,300,000,000	8,200,000,000
Restricted Cash and Cash Equivalents	179,000,000
Investment income	37,000,000us-gaap_InvestmentIncomeNonoperating	69,000,000us-gaap_InvestmentIncomeNonoperating	24,000,000us-gaap_InvestmentIncomeNonoperating
Capitalized computer software costs	501,000,000us-gaap_CapitalizedComputerSoftwareNet	399,000,000us-gaap_CapitalizedComputerSoftwareNet
Capitalized computer software costs accumulated amortization	837,000,000	748,000,000
Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment securities	6,000,000	13,000,000	33,000,000
Trident III [Member] \| Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Recognition of deferred performance fees	31,000,000	41,000,000
Trident III [Member] \| Performance Fees [Member]
Schedule of Equity Method Investments [Line Items]
Deferred Revenue	16,000,000	38,000,000
Trident II [Member] \| Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Recognition of deferred performance fees		15,000,000	0
Mercer Consulting Group [Member]
Schedule of Equity Method Investments [Line Items]
Assets in trusts or funds for management or trustee fee	21,000,000,000
Minimum [Member] \| Software Development [Member]
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	3 years
Maximum [Member] \| Software Development [Member]
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	10 years
Risk and Insurance Services Segment [Member]
Schedule of Equity Method Investments [Line Items]
Interest Income, Fiduciary Assets	24,000,000	27,000,000	39,000,000
Consulting Segment [Member]
Schedule of Equity Method Investments [Line Items]
Number of business groups	2
Interest Income, Fiduciary Assets	$ 6,000,000	$ 5,000,000	$ 4,000,000
Subsequent Event [Member] \| Benefitfocus [Member]
Schedule of Equity Method Investments [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired				10.00%