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, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	$ 776	$ 1,159
Noncurrent liabilities	(2,076)	(2,058)
U.S. Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,685	5,924
Service cost	106	114	$ 91
Interest cost	264	254	253
Employee contributions	0	0
Plan termination	0	0
Actuarial loss (gain)	160	(392)
Benefits paid	(223)	(215)
Benefit obligation at end of year	5,894	5,685	5,924
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	4,160	4,516
Actual return on plan assets	401	(170)
Effect of curtailment	(98)	0
Employer/Company contributions	27	29
Employee contributions	0	0
Benefits paid	(223)	(215)
Other	0	0
Fair value of plan assets at end of year	4,365	4,160	4,516
Funded status	(1,529)	(1,525)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(27)	(26)
Noncurrent liabilities	(1,502)	(1,499)
Net asset (liability) recognized in consolidated balance sheet, December 31	(1,529)	(1,525)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	0
Net actuarial (loss) gain	(1,720)	(1,754)
Total recognized accumulated other comprehensive (loss) income, December 31	(1,720)	(1,754)
Cumulative employer contributions in excess (less than) net periodic cost	191	229
Net asset (liability) recognized in consolidated balance sheet, December 31	(1,529)	(1,525)
Accumulated benefit obligation at December 31	5,894	5,600
U.S. Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	40	177
Service cost	0	1	2
Interest cost	2	2	7
Employee contributions	3	3
Plan termination	0	(128)
Actuarial loss (gain)	0	(5)
Benefits paid	(8)	(10)
Benefit obligation at end of year	37	40	177
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	3	18
Actual return on plan assets	0	0
Effect of curtailment	0	0
Employer/Company contributions	5	4
Employee contributions	3	3
Benefits paid	(8)	(10)
Other	(1)	(12)
Fair value of plan assets at end of year	2	3	18
Funded status	(35)	(37)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	(2)	(2)
Noncurrent liabilities	(33)	(35)
Net asset (liability) recognized in consolidated balance sheet, December 31	(35)	(37)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	(3)	(7)
Net actuarial (loss) gain	11	13
Total recognized accumulated other comprehensive (loss) income, December 31	8	6
Cumulative employer contributions in excess (less than) net periodic cost	(43)	(43)
Net asset (liability) recognized in consolidated balance sheet, December 31	(35)	(37)
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	9,076	10,018
Service cost	72	82	122
Interest cost	273	333	388
Employee contributions	7	8
Plan amendments	(49)	(5)
Actuarial loss (gain)	1,966	(432)
Effect of settlement	(27)	(12)
Benefits paid	(352)	(337)
Foreign currency charges	(1,290)	(632)
Other	1	45
Benefit obligation at end of year	9,670	9,076	10,018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	9,826	10,410
Actual return on plan assets	1,815	187
Effect of settlement	(27)	(12)
Effect of curtailment	(7)	8
Employer/Company contributions	187	166
Employee contributions	7	8
Benefits paid	(352)	(337)
Foreign currency changes	(1,439)	(620)
Other	0	24
Fair value of plan assets at end of year	10,017	9,826	10,410
Funded status	347	750
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	766	1,144
Current liabilities	(5)	(5)
Noncurrent liabilities	(414)	(389)
Net asset (liability) recognized in consolidated balance sheet, December 31	347	750
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	43	(3)
Net actuarial (loss) gain	(3,081)	(2,887)
Total recognized accumulated other comprehensive (loss) income, December 31	(3,038)	(2,890)
Cumulative employer contributions in excess (less than) net periodic cost	3,385	3,640
Net asset (liability) recognized in consolidated balance sheet, December 31	347	750
Accumulated benefit obligation at December 31	9,397	8,830
Non-U.S. Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	79	93
Service cost	2	2	2
Interest cost	3	3	4
Employee contributions	0	0
Plan amendments	0	0
Actuarial loss (gain)	5	(6)
Effect of settlement	0	0
Benefits paid	(3)	(3)
Foreign currency charges	(5)	(10)
Other	0	0
Benefit obligation at end of year	81	79	93
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	(81)	(79)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	0	0
Current liabilities	(3)	(3)
Noncurrent liabilities	(78)	(76)
Net asset (liability) recognized in consolidated balance sheet, December 31	(81)	(79)
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	0
Net actuarial (loss) gain	(11)	(6)
Total recognized accumulated other comprehensive (loss) income, December 31	(11)	(6)
Cumulative employer contributions in excess (less than) net periodic cost	(70)	(73)
Net asset (liability) recognized in consolidated balance sheet, December 31	(81)	(79)
Accumulated benefit obligation at December 31	$ 0	$ 0