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Consolidated Statements of Equity (Unaudited) (Parenthetical) - $ / shares | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Statement of Stockholders' Equity [Abstract] |  |
Dividends declared per share (in dollars per share) | $ 0.75 | $ 0.68
Dividend equivalents declared per share (in dollars per share) | $ 0.75 | $ 0.68

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- Definition Common Stock, Dividends Per Share, Cash, Declared + References No definition available. + Details
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Balance Type: | na
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- Definition Dividend Equivalents Per Share, Cash, Declared + References No definition available. + Details
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Namespace Prefix: | mmc_
Data Type: | num:perShareItemType
Balance Type: | na
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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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