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 ($)	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
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				6.20%
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				5.20%
Ultimate assumed health care cost trend rate				4.48%
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
ESOP invested in MMC common stock				$ 436	$ 398
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net credit due to termination of RRA plan				62	(82)	$ (26)
Curtailment loss (gain)				4	(5)	65
U.S. Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Plans with benefit obligations in excess of plan assets, projected benefit obligation				5,900	5,700
Accumulated benefit obligation				5,900	5,600
Aggregate fair value of plan assets				4,400	4,200
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation				5,900	5,700
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets				4,400	4,200
Net credit due to termination of RRA plan				(65)	(141)	(103)
Pension plan assets				4,365	4,160	4,516
Non-U.S. Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation				2,100	1,900
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets				1,700	1,500
Net credit due to termination of RRA plan				127	59	77
Estimated future contributions (less than $1 million for U.S. tax-qualified plan)				224
Curtailment loss (gain)			$ 65	4	(5)	65
Pension plan assets				10,017	9,826	10,410
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net credit due to termination of RRA plan			$ 125	(9)	118	(14)
Curtailment loss (gain)				$ 0	0	0
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
Health care cost trend rate assumed for next fiscal year				6.90%
Ultimate assumed health care cost trend rate				4.50%
Assumed health care cost trend rate				5.00%
Estimated future contributions (less than $1 million for U.S. tax-qualified plan)				$ 6
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)				32
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				1,200	1,700
Accumulated benefit obligation				1,200	1,600
Aggregate fair value of plan assets				$ 900	1,300
Equity Funds | United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
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]
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 States
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
The cost of defined contribution plans				$ 53	51	49
United States | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation				64.00%
Actual plan asset allocations				65.00%
United States | Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation				36.00%
Actual plan asset allocations				35.00%
United Kingdom
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
The cost of defined contribution plans				$ 81	93	$ 65
United Kingdom | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation				48.00%
Actual plan asset allocations				48.00%
United Kingdom | Equity Funds | Foreign Pension and Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
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
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation				52.00%
Actual plan asset allocations				52.00%
United Kingdom | Fixed Income Funds | Foreign Pension and Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
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%
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				81.00%
Pension Plan Funded Status Guarantee | Non-U.S. Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Maximum exposure under guarantee (up to) | £	£ 450
Guarantor obligation, period	7 years
Accounting Standards Update 2015-07 | Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets					(6,600)
Accounting Standards Update 2015-07 | Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets					$ (1,400)