Retirement Benefits (Schedules Providing Information Concerning MMC's Defined Benefit Pension Plans and Postretirement Benefit Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at end of year	$ 13,562	$ 12,225
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	979	260
Noncurrent liabilities	(1,150)	(2,094)
U.S. Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,197	4,533
Service cost	104	93	83
Interest cost	229	230	231
Plan combination	36
Actuarial (gain) loss	(547)	522
Medicare Part D subsidy	0	0
Benefits paid	(192)	(181)
Benefit obligation at end of year	4,827	5,197	4,533
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	3,936	3,493
Plan Combination	21
Actual return on plan assets	488	500
Employer/Company contributions	26	124
Benefits paid	(192)	(181)
Fair value of plan assets at end of year	4,279	3,936	3,493
Funded status	(548)	(1,261)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(24)	(25)
Noncurrent liabilities	(524)	(1,236)
Net liability recognized in consolidated balance sheet, December 31	(548)	(1,261)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	7	23
Net actuarial (loss) gain	(974)	(1,887)
Total recognized accumulated other comprehensive (loss) income, December 31	(967)	(1,864)
Cumulative employer contributions in excess (deficient) of net periodic cost	419	603
Net liability recognized in consolidated balance sheet, December 31	(548)	(1,261)
Accumulated benefit obligation at December 31	4,753	5,114
U.S. Postretirement Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	176	162
Service cost	3	3	3
Interest cost	7	8	8
Actuarial (gain) loss	(15)	13
Medicare Part D subsidy	1	3
Benefits paid	(14)	(13)
Benefit obligation at end of year	158	176	162
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer/Company contributions	13	10
Benefits paid	(14)	(13)
Fair value of plan assets at end of year	0	0	0
Funded status	(158)	(176)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(8)	(9)
Noncurrent liabilities	(150)	(167)
Net liability recognized in consolidated balance sheet, December 31	(158)	(176)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	0
Net actuarial (loss) gain	13	(2)
Total recognized accumulated other comprehensive (loss) income, December 31	13	(2)
Cumulative employer contributions in excess (deficient) of net periodic cost	(171)	(174)
Net liability recognized in consolidated balance sheet, December 31	(158)	(176)
Accumulated benefit obligation at December 31	0	0
Non- U.S Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	8,579	7,717
Service cost	148	147	143
Interest cost	352	366	378
Employee contributions	11	11
Plan combination	0	(71)
Actuarial (gain) loss	(53)	419
Effect of Settlement	(2)	(11)
Effect of curtailment	0	(3)
Benefits paid	(293)	(278)
Foreign Currency Charges	(31)	280
Other	0	2
Benefit obligation at end of year	8,711	8,579	7,717
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	8,312	7,206
Actual return on plan assets	698	721
Effect of settlement	(2)	(11)
Employer/Company contributions	620	389
Employee contributions	11	11
Benefits paid	(293)	(278)
Foreign currency changes	5	273
Other	0	1
Fair value of plan assets at end of year	9,351	8,312	7,206
Funded status	640	(267)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	977	258
Current liabilities	(5)	(6)
Noncurrent liabilities	(332)	(519)
Net liability recognized in consolidated balance sheet, December 31	640	(267)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	85	93
Net actuarial (loss) gain	(3,010)	(3,309)
Total recognized accumulated other comprehensive (loss) income, December 31	(2,925)	(3,216)
Cumulative employer contributions in excess (deficient) of net periodic cost	3,565	2,949
Net liability recognized in consolidated balance sheet, December 31	640	(267)
Accumulated benefit obligation at December 31	8,413	8,229
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	107	91
Service cost	2	2	2
Interest cost	4	5	5
Employee contributions	0	0
Plan combination	0	0
Actuarial (gain) loss	(8)	10
Effect of Settlement	0	0
Effect of curtailment	0	(1)
Benefits paid	(4)	(4)
Foreign Currency Charges	(4)	4
Other	0	0
Benefit obligation at end of year	97	107	91
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Effect of settlement	0	0
Employer/Company contributions	4	4
Employee contributions	0	0
Benefits paid	(4)	(4)
Foreign currency changes	0	0
Other	0	0
Fair value of plan assets at end of year	0	0	0
Funded status	(97)	(107)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	0	0
Current liabilities	(4)	(4)
Noncurrent liabilities	(93)	(103)
Net liability recognized in consolidated balance sheet, December 31	(97)	(107)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	0
Net actuarial (loss) gain	(16)	(27)
Total recognized accumulated other comprehensive (loss) income, December 31	(16)	(27)
Cumulative employer contributions in excess (deficient) of net periodic cost	(81)	(80)
Net liability recognized in consolidated balance sheet, December 31	(97)	(107)
Accumulated benefit obligation at December 31	$ 0	$ 0