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Retirement Benefits (Narrative) (Details) (USD $)
Share data in Millions, unless otherwise specified
 12 Months Ended 12 Months Ended
Dec. 31, 2011
Dec. 31, 2009
Dec. 31, 2012
Dec. 31, 2010
Dec. 31, 2011
Non Medicare Eligible [Member]
Dec. 31, 2011
Medicare Eligible [Member]
Dec. 31, 2011
U.S. Pension Benefits [Member]
Dec. 31, 2010
U.S. Pension Benefits [Member]
Dec. 31, 2011
Non- U.S Pension Benefits [Member]
Dec. 31, 2010
Non- U.S Pension Benefits [Member]
Dec. 31, 2011
U.S. Postretirement Benefits [Member]
Dec. 31, 2010
U.S. Postretirement Benefits [Member]
Dec. 31, 2011
Non- U.S Postretirement Benefits [Member]
Dec. 31, 2010
Non- U.S Postretirement Benefits [Member]
Dec. 31, 2011
U.S. Plans [Member]
Dec. 31, 2011
U.K. Plan [Member]
Dec. 31, 2011
Non-U.S. Plans [Member]
Dec. 31, 2010
Non-U.S. Plans [Member]
Target asset allocation U.S. Plan, equities, prior period                             58.00%      
Target asset allocation U.S. Plan, fixed income, prior period                             42.00%      
Target asset allocation percentage for non-U.S. plan assets                                 82.00%  
Actual asset allocation percentage of equity                             58.00% 53.00%    
Actual asset allocation percentage of fixed income                             42.00% 47.00%    
Common Stock Shares Held By Defined Benefit Plan                             8      
Target asset allocation non-U.S. Plan, equities                               58.00%    
Target asset allocation non-U.S. Plan, fixed income                               42.00%    
Shares of common stock represnting percentage of plan assets 7.00%                                  
Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum                             53.00% 55.00%    
Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum                             63.00% 61.00%    
Defined benefit plan target allocation percentage of assets fixed income securities range, minimum                             37.00% 39.00%    
Defined benefit plan target allocation percentage of assets fixed income securities range, maximum                             47.00% 45.00%    
The projected benefit obligation with accumulated benefit obligations             $ 4,500,000,000 $ 4,000,000,000                 $ 1,400,000,000 $ 1,200,000,000
Accumulated benefit obligation             4,500,000,000 3,900,000,000                 1,300,000,000 1,100,000,000
Aggregate fair value of plan assets             3,500,000,000 3,400,000,000                 1,000,000,000 900,000,000
The projected benefit obligation             4,500,000,000 4,000,000,000 7,600,000,000 1,300,000,000                
Fair value of plan assets with projected benefit obligation             3,500,000,000 3,400,000,000 7,000,000,000 1,000,000,000                
Defined benefit plan, health care cost trend rate assumed for next fiscal year         8.00% 8.10%                        
Assumed health care cost trend rate 6.70%       4.50% 4.50%                 5.00%      
Ultimate assumed health care cost trend rate 4.90%                                  
Defined Benefit Plan, Contributions by Employer 100,000,000           24,000,000 221,000,000 320,000,000 237,000,000 9,000,000 8,000,000 4,000,000 3,000,000        
Expected fund in U.S. non qualified plan 24,000,000                                  
Limited partnership interest in private equity fund 169,000,000                                  
Expected fund in non U.S. pension plans     395,000,000                              
ESOP invested in MMC common stock 366,000,000     334,000,000                            
The cost of defined contribution plans $ 48,000,000 $ 52,000,000