XML 75 R63.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Derivatives - Narrative (Details) $ in Millions |  | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 18, 2018 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Jun. 29, 2019 | Mar. 30, 2019
Derivative [Line Items] |  |  |  |  |
Total consideration |  | $ 661 | $ 734 |  |
Exchange rate |  | 0.0343 |  |  |
Foreign Exchange Forward |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Unrealized loss |  | $ 100 |  |  |
Jardine Lloyd Thompson Group plc |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Total consideration | $ 5,200 |  |  |  |
Scenario, Forecast [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Exchange rate |  |  |  | 0.000145 | 0.00016

X
- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=109124213&loc=d3e6613-128477 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=109124213&loc=d3e6578-128477 + Details
Name: | us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Contractual rate at which a foreign currency can be purchased or sold. + References No definition available. + Details
Name: | us-gaap_DerivativeForwardExchangeRate1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition Amount of decrease in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 + Details
Name: | us-gaap_DerivativeLossOnDerivative
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=mmc_JardineLloydThompsonGroupplcMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |