Retirement Benefits (Schedules Providing Information Concerning MMC's Defined Benefit Pension Plans and Postretirement Benefit Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 14,879
Fair value of plan assets at end of year	13,972	$ 14,879
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	1,159	967
Noncurrent liabilities	(2,058)	(2,244)
U.S. Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,924	4,827
Service cost	114	91	$ 104
Interest cost	254	253	229
Employee contributions	0	0
Plan amendments	0	0
Plan combination	0	0
Plan termination	0	0
Actuarial (gain) loss	(392)	955
Medicare Part D subsidy	0	0
Benefits paid	(215)	(202)
Benefit obligation at end of year	5,685	5,924	4,827
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	4,516	4,279
Actual return on plan assets	(170)	414
Employer/Company contributions	29	25
Employee contributions	0	0
Benefits paid	(215)	(202)
Other	0	0
Fair value of plan assets at end of year	4,160	4,516	4,279
Funded status	(1,525)	(1,408)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(26)	(25)
Noncurrent liabilities	(1,499)	(1,383)
Net asset (liability) recognized in consolidated balance sheet, December 31	(1,525)	(1,408)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	0
Net actuarial (loss) gain	(1,754)	(1,749)
Total recognized accumulated other comprehensive (loss) income, December 31	(1,754)	(1,749)
Cumulative employer contributions in excess (deficient) of net periodic cost	229	341
Net asset (liability) recognized in consolidated balance sheet, December 31	(1,525)	(1,408)
Accumulated benefit obligation at December 31	5,600	5,825
U.S. Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	177	158
Service cost	1	2	3
Interest cost	2	7	7
Employee contributions	3	13
Plan amendments	0	(4)
Plan combination	0	0
Plan termination	(128)	0
Actuarial (gain) loss	(5)	21
Medicare Part D subsidy	0	1
Benefits paid	(10)	(21)
Benefit obligation at end of year	40	177	158
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	18	0
Actual return on plan assets	0	0
Employer/Company contributions	4	13
Employee contributions	3	13
Benefits paid	(10)	(21)
Other	(12)	12
Fair value of plan assets at end of year	3	18	0
Funded status	(37)	(159)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(2)	(2)
Noncurrent liabilities	(35)	(157)
Net asset (liability) recognized in consolidated balance sheet, December 31	(37)	(159)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	(7)	4
Net actuarial (loss) gain	13	2
Total recognized accumulated other comprehensive (loss) income, December 31	6	6
Cumulative employer contributions in excess (deficient) of net periodic cost	(43)	(165)
Net asset (liability) recognized in consolidated balance sheet, December 31	(37)	(159)
Accumulated benefit obligation at December 31	0	0
Non-U.S. Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	10,018	8,711
Service cost	82	122	148
Interest cost	333	388	352
Employee contributions	8	10
Plan amendments	(5)	13
Actuarial (gain) loss	(432)	1,619
Effect of settlement	(12)	(11)
Benefits paid	(337)	(311)
Foreign currency charges	(632)	(585)
Other	45	62
Benefit obligation at end of year	9,076	10,018	8,711
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	10,410	9,351
Actual return on plan assets	187	1,756
Effect of settlement	(12)	(11)
Effect of curtailment	8	0
Employer/Company contributions	166	156
Employee contributions	8	10
Benefits paid	(337)	(311)
Foreign currency changes	(620)	(578)
Other	24	37
Fair value of plan assets at end of year	9,826	10,410	9,351
Funded status	750	392
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	1,144	967
Current liabilities	(5)	(6)
Noncurrent liabilities	(389)	(569)
Net asset (liability) recognized in consolidated balance sheet, December 31	750	392
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	(3)	(2)
Net actuarial (loss) gain	(2,887)	(3,215)
Total recognized accumulated other comprehensive (loss) income, December 31	(2,890)	(3,217)
Cumulative employer contributions in excess (deficient) of net periodic cost	3,640	3,609
Net asset (liability) recognized in consolidated balance sheet, December 31	750	392
Accumulated benefit obligation at December 31	8,830	9,731
Non-U.S. Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	93	97
Service cost	2	2	2
Interest cost	3	4	4
Employee contributions	0	0
Plan amendments	0	0
Actuarial (gain) loss	(6)	(1)
Effect of settlement	0	0
Benefits paid	(3)	(3)
Foreign currency charges	(10)	(6)
Other	0	0
Benefit obligation at end of year	79	93	97
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
Effect of curtailment	0	0
Employer/Company contributions	3	3
Employee contributions	0	0
Benefits paid	(3)	(3)
Foreign currency changes	0	0
Other	0	0
Fair value of plan assets at end of year	0	0	$ 0
Funded status	(79)	(93)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	0	0
Current liabilities	(3)	(4)
Noncurrent liabilities	(76)	(89)
Net asset (liability) recognized in consolidated balance sheet, December 31	(79)	(93)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	0
Net actuarial (loss) gain	(6)	(14)
Total recognized accumulated other comprehensive (loss) income, December 31	(6)	(14)
Cumulative employer contributions in excess (deficient) of net periodic cost	(73)	(79)
Net asset (liability) recognized in consolidated balance sheet, December 31	(79)	(93)
Accumulated benefit obligation at December 31	$ 0	$ 0