XML 20 R8.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Consolidated Statements of Equity (Unaudited) (Parenthetical) - $ / shares | 9 Months Ended
--- | ---
Sep. 30, 2018 | Sep. 30, 2017
Statement of Stockholders' Equity [Abstract] |  |
Dividends declared per share (in dollars per share) | $ 1.58 | $ 1.43
Dividend equivalents declared per share (in dollars per share) | $ 1.58 | $ 1.43

X
- Definition Common Stock, Dividends Per Share, Cash, Declared + References No definition available. + Details
Name: | mmc_CommonStockDividendsPerShareCashDeclared
Namespace Prefix: | mmc_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- Definition Dividend Equivalents Per Share, Cash, Declared + References No definition available. + Details
Name: | mmc_DividendEquivalentsPerShareCashDeclared
Namespace Prefix: | mmc_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_StatementOfStockholdersEquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration