Retirement Benefits (Narrative) (Details) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended		12 Months Ended					12 Months Ended						12 Months Ended			12 Months Ended														3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Non Medicare Eligible [Member]	Dec. 31, 2013 Medicare Eligible [Member]	Dec. 31, 2013 U.S. Pension Benefits [Member]	Dec. 31, 2012 U.S. Pension Benefits [Member]	Dec. 31, 2011 U.S. Pension Benefits [Member]	Dec. 31, 2013 Non- U.S Pension Benefits [Member]	Dec. 31, 2012 Non- U.S Pension Benefits [Member]	Dec. 31, 2011 Non- U.S Pension Benefits [Member]	Dec. 31, 2013 U.S. Postretirement Benefits [Member]	Dec. 31, 2012 U.S. Postretirement Benefits [Member]	Dec. 31, 2011 U.S. Postretirement Benefits [Member]	Dec. 31, 2013 United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]	Dec. 31, 2013 Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2012 Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2013 United States Nonqualified Pension Plan of US Entity, Defined Benefit [Member]	Dec. 31, 2013 Equity Funds [Member] United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]	Dec. 31, 2013 Fixed Income Funds [Member] United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]	Dec. 31, 2013 United Kingdom [Member]	Dec. 31, 2012 United Kingdom [Member]	Dec. 31, 2011 United Kingdom [Member]	Dec. 31, 2013 United Kingdom [Member] Equity Funds [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2013 United Kingdom [Member] Fixed Income Funds [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2013 United States [Member]	Dec. 31, 2012 United States [Member]	Dec. 31, 2011 United States [Member]	Dec. 31, 2013 Geographic Concentration Risk [Member] United Kingdom [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member] Non- U.S Pension Benefits [Member]	Dec. 31, 2013 Trident III [Member] United Kingdom [Member] Equity Funds [Member] Non- U.S Pension Benefits [Member]	Jan. 31, 2014 Subsequent Event [Member] Non- U.S Pension Benefits [Member]	Mar. 31, 2014 Scenario, Forecast [Member] Subsequent Event [Member] Non- U.S Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Market-related plan assets value, recognition period of investment gains and losses	5 years
Target asset allocation U.S. Plan, equities, prior period																		58.00%	42.00%				50.00%	50.00%
Actual plan asset allocations																		63.00%	37.00%				49.00%	51.00%
Target asset allocation percentage for non-U.S. plan assets, U.K. Plans																												82.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum																		53.00%	37.00%				47.00%	47.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum																		63.00%	47.00%				53.00%	53.00%
The projected benefit obligation with accumulated benefit obligations					$ 4,800,000,000	$ 5,200,000,000									$ 1,700,000,000	$ 1,700,000,000
Accumulated benefit obligation					4,800,000,000	5,100,000,000									1,500,000,000	1,600,000,000
Aggregate fair value of plan assets					4,300,000,000	3,900,000,000									1,300,000,000	1,200,000,000
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation					4,800,000,000	5,200,000,000		1,700,000,000	1,700,000,000
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets					4,300,000,000	3,900,000,000		1,300,000,000	1,200,000,000
Effect of plan amendment on projected benefit obligation																														147,000,000
Effect of plan amendment on funded status																														137,000,000
Plan amendment curtailment gain																															63,000,000
Number of shares the U.S. qualified plan holds contributed by the Company														5.4
Shares of common stock representing percentage of plan assets	6.00%
Health care cost trend rate assumed for next fiscal year	5.80%		7.50%	7.60%
Ultimate assumed health care cost trend rate	4.85%		4.50%	4.50%
Assumed health care cost trend rate														5.00%
Estimated future contributions (less than $1 million for U.S. tax-qualified plan)					1,000,000			160,000,000									24,000,000
Limited partnership interest in private equity fund	13,562,000,000	12,225,000,000			4,279,000,000	3,936,000,000	3,493,000,000	9,351,000,000	8,312,000,000	7,206,000,000	0	0	0																111,000,000
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
ESOP invested in MMC common stock	453,000,000	375,000,000
The cost of defined contribution plans																				$ 23,000,000	$ 21,000,000	$ 20,000,000			$ 50,000,000	$ 50,000,000	$ 48,000,000