Per Share Data (Basic and Diluted EPS Calculation Continuing Operations) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Net income from continuing operations	$ 498	$ 457
Less: Net income attributable to non-controlling interests	13	13
Net income attributable to the Company	$ 485	$ 444
Basic weighted average common shares outstanding (in shares)	539	548
Dilutive effect of potentially issuable common shares (in shares)	6	8
Diluted weighted average common shares outstanding (in shares)	545	556
Average stock price used to calculate common stock equivalents (in dollars per share)	$ 56.37mmc_AverageSharePriceCommonStock	$ 47.84mmc_AverageSharePriceCommonStock
Stock options outstanding (in shares)	17.2	21.6

Average stock price used to calculate common stock equivalents

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Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

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Amount of income (loss) from continuing operations attributable to the noncontrolling interest. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and noncontrolling interest.

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

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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