Retirement Benefits (Narrative) (Details) Share data in Millions, unless otherwise specified	12 Months Ended					12 Months Ended							12 Months Ended			12 Months Ended													1 Months Ended		12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2014 U.S. Pension Benefits [Member] USD ($)	Dec. 31, 2013 U.S. Pension Benefits [Member] USD ($)	Dec. 31, 2012 U.S. Pension Benefits [Member] USD ($)	Dec. 31, 2014 Non- U.S Pension Benefits [Member] USD ($)	Jan. 31, 2014 Non- U.S Pension Benefits [Member] USD ($)	Dec. 31, 2013 Non- U.S Pension Benefits [Member] USD ($)	Dec. 31, 2012 Non- U.S Pension Benefits [Member] USD ($)	Dec. 31, 2014 U.S. Postretirement Benefits [Member] USD ($)	Dec. 31, 2013 U.S. Postretirement Benefits [Member] USD ($)	Dec. 31, 2012 U.S. Postretirement Benefits [Member] USD ($)	Dec. 31, 2014 United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]	Dec. 31, 2014 Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member] USD ($)	Dec. 31, 2013 Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member] USD ($)	Dec. 31, 2014 United States Nonqualified Pension Plan of US Entity, Defined Benefit [Member] USD ($)	Dec. 31, 2014 Equity Funds [Member] United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]	Dec. 31, 2014 Fixed Income Funds [Member] United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]	Dec. 31, 2014 United Kingdom [Member] USD ($)	Dec. 31, 2013 United Kingdom [Member] USD ($)	Dec. 31, 2012 United Kingdom [Member] USD ($)	Dec. 31, 2014 United Kingdom [Member] Equity Funds [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2014 United Kingdom [Member] Fixed Income Funds [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2014 United States [Member] USD ($)	Dec. 31, 2013 United States [Member] USD ($)	Dec. 31, 2012 United States [Member] USD ($)	Dec. 31, 2014 Geographic Concentration Risk [Member] United Kingdom [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member] Non- U.S Pension Benefits [Member]	Dec. 31, 2014 Trident III [Member] United Kingdom [Member] Equity Funds [Member] Non- U.S Pension Benefits [Member] USD ($)	Feb. 26, 2015 Subsequent Event [Member] United Kingdom [Member] Equity Funds [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Feb. 26, 2015 Subsequent Event [Member] United Kingdom [Member] Fixed Income Funds [Member] Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2014 Pension Plan Funded Status Guarantee [Member] Non- U.S Pension Benefits [Member] GBP (£)
Defined Benefit Plan Disclosure [Line Items]
Market-related plan assets value, recognition period of investment gains and losses	5 years
Actual plan asset allocations																	59.00%	41.00%				43.00%	57.00%
Target asset allocation percentage for non-U.S. plan assets, U.K. Plans																											83.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum																	57.00%	33.00%				47.00%	47.00%
Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum																	67.00%	43.00%				53.00%	53.00%
Target asset allocation																	62.00%	38.00%				50.00%	50.00%						48.00%	52.00%
The projected benefit obligation with accumulated benefit obligations			$ 5,900,000,000	$ 4,800,000,000										$ 2,100,000,000	$ 1,700,000,000
Accumulated benefit obligation			5,800,000,000	4,800,000,000										2,000,000,000	1,500,000,000
Aggregate fair value of plan assets			4,500,000,000	4,300,000,000										1,600,000,000	1,300,000,000
Plans with benefit obligations in excess of plan assets, aggregate benefit obligation			5,900,000,000	4,800,000,000		2,200,000,000		1,700,000,000
Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets			4,500,000,000	4,300,000,000		1,600,000,000		1,300,000,000
Number of shares the U.S. qualified plan holds contributed by the Company													4.0
Shares of common stock representing percentage of plan assets	5.00%
Health care cost trend rate assumed for next fiscal year	5.76%												7.55%
Ultimate assumed health care cost trend rate	4.92%												4.50%
Assumed health care cost trend rate													5.00%
Estimated future contributions (less than $1 million for U.S. tax-qualified plan)						169,000,000										25,000,000
Limited partnership interest in private equity fund	14,879,000,000	13,562,000,000	4,516,000,000	4,279,000,000	3,936,000,000	10,410,000,000		9,351,000,000	8,312,000,000	18,000,000	0	0																53,000,000
Effect of plan amendment on projected benefit obligation							147,000,000
Effect of plan amendment on funded status							137,000,000
Maximum exposure under guarantee																															450,000,000
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
ESOP invested in MMC common stock	453,000,000	453,000,000
The cost of defined contribution plans																			$ 65,000,000	$ 23,000,000	$ 21,000,000			$ 49,000,000	$ 50,000,000	$ 50,000,000
Guarantor Obligation, Period																															7 years