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Retirement Benefits (Narrative) (Details) £ in Millions, $ in Millions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2017 | Nov. 30, 2016 GBP (£) | Oct. 31, 2016 | Dec. 31, 2018 USD ($) shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Market-related plan assets value, recognition period of investment gains and losses |  |  |  | 5 years |  |
Plans with benefit obligations in excess of plan assets, projected benefit obligation |  |  |  | $ 1,700 | $ 1,300 |
Accumulated benefit obligation |  |  |  | 1,600 | 1,200 |
Aggregate fair value of plan assets, accumulated benefit obligation |  |  |  | 1,300 | 1,000 |
Projected benefit obligation |  |  |  | 1,900 | 2,200 |
Aggregate fair value of plan assets, projected benefit obligation |  |  |  | $ 1,600 | 1,900 |
Employer matching contribution, percent of employees' gross pay (up to) | 4.00% |  | 4.00% |  |  |
Maximum annual contributions per employee, percent |  |  | 7.00% |  |  |
Health care cost trend rate assumed for next fiscal year |  |  |  | 5.09% |  |
Ultimate assumed health care cost trend rate |  |  |  | 4.46% |  |
Defined Contribution Plan [Abstract] |  |  |  |  |  |
ESOP invested in MMC common stock |  |  |  | $ 444 | 499 |
Geographic Concentration Risk | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Pension Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation percentage for non-U.S. plan assets, U.K. Plans |  |  |  | 81.00% |  |
Foreign Plan | Pension Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Settlement charge |  |  |  | $ 42 | 54 |
Estimated future contributions (less than $1 million for U.S. tax-qualified plan) |  |  |  | 65 |  |
Pension plan assets |  |  |  | 10,306 | 11,388 | $ 10,017
Foreign Plan | Postretirement Benefits |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Pension plan assets |  |  |  | $ 0 | 0 | 0
Foreign Plan | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 34.00% |  |
Foreign Plan | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 66.00% |  |
United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 64.00% |  |
United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 36.00% |  |
Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 34.00% |  |
Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 66.00% |  |
United States |  |  |  |  |  |
Defined Contribution Plan [Abstract] |  |  |  |  |  |
The cost of defined contribution plans |  |  |  | $ 133 | 130 | 53
United States | Pension Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Plans with benefit obligations in excess of plan assets, projected benefit obligation |  |  |  | 5,500 | 6,200 |
Accumulated benefit obligation |  |  |  | 5,500 | 6,200 |
Aggregate fair value of plan assets, accumulated benefit obligation |  |  |  | 4,100 | 4,800 |
Projected benefit obligation |  |  |  | 5,500 | 6,200 |
Pension plan assets |  |  |  | $ 4,062 | 4,787 | 4,365
United States | Postretirement Benefits |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Number of shares the U.S. qualified plan holds contributed by the Company | shares |  |  |  | 4,000,000 |  |
Shares of common stock representing percentage of plan assets |  |  |  | 7.90% |  |
Pension plan assets |  |  |  | $ 1 | 2 | 2
United States | Pension and Postretirement Benefit Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Health care cost trend rate assumed for next fiscal year |  |  |  | 6.19% |  |
Ultimate assumed health care cost trend rate |  |  |  | 4.50% |  |
Assumed health care cost trend rate |  |  |  | 5.00% |  |
United States | Nonqualified Pension Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Estimated future contributions (less than $1 million for U.S. tax-qualified plan) |  |  |  | $ 28 |  |
United States | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 62.00% |  |
United States | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 38.00% |  |
United Kingdom |  |  |  |  |  |
Defined Contribution Plan [Abstract] |  |  |  |  |  |
The cost of defined contribution plans |  |  |  | $ 80 | $ 75 | $ 81
Minimum | United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 59.00% |  |
Minimum | United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 31.00% |  |
Minimum | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 31.00% |  |
Minimum | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 63.00% |  |
Maximum | United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 69.00% |  |
Maximum | United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 41.00% |  |
Maximum | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Equity Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 37.00% |  |
Maximum | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Target asset allocation |  |  |  | 69.00% |  |
Pension Plan Funded Status Guarantee | Foreign Plan |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Maximum exposure under guarantee (up to) | £ |  | £ 450 |  |  |  |
Guarantor obligation, period |  | 7 years |  |  |  |

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