Retirement Benefits (Narrative) (Details) £ in Millions, $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Feb. 26, 2015	Mar. 31, 2015 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 GBP (£) shares
Defined Benefit Plan Disclosure [Line Items]
Market-related plan assets value, recognition period of investment gains and losses					5 years
Shares of common stock representing percentage of plan assets					5.30%
Health care cost trend rate assumed for next fiscal year					5.30%
Ultimate assumed health care cost trend rate					4.53%
Limited partnership interest in private equity fund					$ 13,972	$ 14,879
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
ESOP invested in MMC common stock					398	453
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net credit due to termination of RRA plan					82	26	$ 215
Curtailment loss (gain)					(5)	65	0
U.S. Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Plans with benefit obligations in excess of plan assets, projected benefit obligation					5,700	5,900
Accumulated benefit obligation					5,600	5,800
Aggregate fair value of plan assets					4,200	4,500
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation					5,700	5,900
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets					4,200	4,500
Net credit due to termination of RRA plan					141	103	200
Limited partnership interest in private equity fund					4,160	4,516	4,279
Non-U.S. Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation					1,900	2,200
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets					1,500	1,600
Net credit due to termination of RRA plan					(59)	(77)	15
Curtailment loss (gain)			$ 65		(5)	65	0
Estimated future contributions (less than $1 million for U.S. tax-qualified plan)					191
Limited partnership interest in private equity fund					9,826	10,410	9,351
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net credit due to termination of RRA plan		$ (125)			(118)	14	18
Curtailment loss (gain)					0	0	0
U.S. Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net credit due to termination of RRA plan					(124)	7	10
Limited partnership interest in private equity fund					3	18	0
Foreign Pension and Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plans with benefit obligations in excess of plan assets, projected benefit obligation					1,700	2,100
Accumulated benefit obligation					1,600	2,000
Aggregate fair value of plan assets					1,300	1,600
United States Nonqualified Pension Plan of US Entity, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Estimated future contributions (less than $1 million for U.S. tax-qualified plan)					$ 26
United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Number of shares the U.S. qualified plan holds contributed by the Company | shares					4,000,000			4,000,000
Health care cost trend rate assumed for next fiscal year					7.30%
Ultimate assumed health care cost trend rate					4.50%
Assumed health care cost trend rate					5.00%
Equity Funds | United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation					64.00%
Actual plan asset allocations					63.00%			63.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum					59.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum					69.00%
Fixed Income Funds | United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation					36.00%
Actual plan asset allocations					37.00%			37.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum					31.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum					41.00%
United Kingdom
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
The cost of defined contribution plans					$ 93	65	23
United Kingdom | Equity Funds | Foreign Pension and Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation	48.00%				48.00%
Actual plan asset allocations					47.00%			47.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum					45.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum					51.00%
United Kingdom | Fixed Income Funds | Foreign Pension and Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation	52.00%				52.00%
Actual plan asset allocations					53.00%			53.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum					49.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum					55.00%
United States
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
The cost of defined contribution plans					$ 51	$ 49	$ 50
Geographic Concentration Risk | United Kingdom | Foreign Pension and Postretirement Benefit Plan, Defined Benefit | Non-U.S. Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage for non-U.S. plan assets, U.K. Plans					83.00%
Pension Plan Funded Status Guarantee | Non-U.S. Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Maximum exposure under guarantee | £								£ 450
Guarantor obligation, period					7 years
Scenario, Forecast | Change in Assumptions for Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, change in estimate of interest and service cost				$ 45