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Retirement Benefits (Narrative) (Details) £ in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Nov. 30, 2016 GBP (£) | Oct. 31, 2016 | Mar. 31, 2015 USD ($) | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2017
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 |  |  |  | $ 1,300 | $ 1,200 |  |
Accumulated benefit obligation |  |  |  | 1,200 | 1,200 |  |
Aggregate fair value of plan assets |  |  |  | 1,000 | 900 |  |
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation |  |  |  | 2,200 | 2,100 |  |
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets |  |  |  | $ 1,900 | 1,700 |  |
Employer matching contribution, percent of employees' gross pay (up to) |  | 4.00% |  |  |  |  |
Maximum annual contributions per employee, percent |  | 7.00% |  |  |  |  |
Health care cost trend rate assumed for next fiscal year |  |  |  | 5.12% |  |  |
Ultimate assumed health care cost trend rate |  |  |  | 4.41% |  |  |
Defined Contribution Plan [Abstract] |  |  |  |  |  |  |
ESOP invested in MMC common stock |  |  |  | $ 499 | 436 |  |
Pension Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Net credit due to termination of RRA plan |  |  |  | 130 | 62 | $ (82) |
Curtailment loss (gain) |  |  |  | 1 | 4 | (5) |
Postretirement Benefits |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Net credit due to termination of RRA plan |  |  | $ 125 | (6) | (9) | 118 |
Curtailment loss (gain) |  |  |  | 0 | 0 | 0 |
United States |  |  |  |  |  |  |
Defined Contribution Plan [Abstract] |  |  |  |  |  |  |
The cost of defined contribution plans |  |  |  | 130 | 53 | 51 |
United Kingdom |  |  |  |  |  |  |
Defined Contribution Plan [Abstract] |  |  |  |  |  |  |
The cost of defined contribution plans |  |  |  | $ 75 | 81 | 93 |
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% |  |  |
Domestic Plan | Pension Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Plans with benefit obligations in excess of plan assets, projected benefit obligation |  |  |  | $ 6,200 | 5,900 |  |
Accumulated benefit obligation |  |  |  | 6,200 | 5,900 |  |
Aggregate fair value of plan assets |  |  |  | 4,800 | 4,400 |  |
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation |  |  |  | 6,200 | 5,900 |  |
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets |  |  |  | 4,800 | 4,400 |  |
Net credit due to termination of RRA plan |  |  |  | 56 | (65) | (141) |
Pension plan assets |  |  |  | $ 4,787 | 4,365 | 4,160 |
Domestic Plan | 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 |  |  |  | 6.80% |  |  |
Net credit due to termination of RRA plan |  |  |  | $ (4) | (4) | 124 |
Pension plan assets |  |  |  | $ 2 | 2 | 3 |
Domestic Plan | Pension and Postretirement Benefit Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Health care cost trend rate assumed for next fiscal year |  |  |  | 6.38% |  |  |
Ultimate assumed health care cost trend rate |  |  |  | 4.50% |  |  |
Assumed health care cost trend rate |  |  |  | 5.00% |  |  |
Domestic Plan | Nonqualified Pension Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Estimated future contributions (less than $1 million for U.S. tax-qualified plan) |  |  |  | $ 27 |  |  |
Domestic Plan | Equity Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 63.00% |  |  |
Domestic Plan | Fixed Income Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 37.00% |  |  |
Foreign Plan | Pension Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Settlement charge |  |  |  | $ 54 |  |  |
Net credit due to termination of RRA plan |  |  |  | 74 | 127 | 59 |
Estimated future contributions (less than $1 million for U.S. tax-qualified plan) |  |  |  | 82 |  |  |
Curtailment loss (gain) |  |  |  | 1 | 4 | (5) |
Pension plan assets |  |  |  | 11,388 | 10,017 | 9,826 |
Foreign Plan | Postretirement Benefits |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Net credit due to termination of RRA plan |  |  |  | (2) | (5) | (6) |
Curtailment loss (gain) |  |  |  | 0 | 0 | 0 |
Pension plan assets |  |  |  | $ 0 | $ 0 | $ 0 |
Foreign Plan | Equity Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 48.00% |  |  |
Foreign Plan | Fixed Income Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Actual plan asset allocations |  |  |  | 52.00% |  |  |
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%
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 | United Kingdom | Equity Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Target asset allocation |  |  |  | 35.00% |  |  |
Minimum | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | United Kingdom | Fixed Income Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Target asset allocation |  |  |  | 59.00% |  |  |
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 | United Kingdom | Equity Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Target asset allocation |  |  |  | 41.00% |  |  |
Maximum | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | United Kingdom | Fixed Income Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Target asset allocation |  |  |  | 65.00% |  |  |
Adjustment | Foreign Plan | Equity Funds |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Target asset allocation |  |  |  | 34.00% |  |  |
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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