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