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, 2011	Dec. 31, 2010	Dec. 31, 2009
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Employer/Company contributions	$ 100
Noncurrent assets	39	265
Noncurrent liabilities	(1,655)	(1,211)
Total amounts included in accumulated other comprehensive income	3,152	2,373
U.S. Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	4,041	3,629
Service cost	83	76	76
Interest cost	231	227	219
Employer/Company contributions	24	221
Actuarial (gain) loss	352	270
Medicare Part D subsidy	0	0
Benefits paid	(174)	(161)
Benefit obligation at end of year	4,533	4,041	3,629
Fair value of plan assets at beginning of year	3,444	2,959
Actual return on plan assets	199	425
Fair value of plan assets at end of year	3,493	3,444	2,959
Funded status	(1,040)	(597)
Current liabilities	(124)	(23)
Noncurrent liabilities	(916)	(574)
Unrecognized prior service credit	39	55	73
Unrecognized net actuarial (loss) gain	(1,695)	(1,327)	(1,258)
Total amounts included in accumulated other comprehensive income	(1,656)	(1,272)
Cumulative employer contributions in excess of net periodic cost	616	675
Net (liability) asset recognized	(1,040)	(597)
Accumulated benefit obligation at December 31	4,467	3,929
U.S. Postretirement Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	180	163
Service cost	3	3	4
Interest cost	8	10	12
Employer/Company contributions	9	8
Actuarial (gain) loss	(20)	12
Medicare Part D subsidy	4	3
Benefits paid	(13)	(11)
Benefit obligation at end of year	162	180	163
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Fair value of plan assets at end of year	0	0	0
Funded status	(162)	(180)
Current liabilities	(9)	(11)
Noncurrent liabilities	(153)	(169)
Unrecognized prior service credit	13	26	39
Unrecognized net actuarial (loss) gain	12	(4)	9
Total amounts included in accumulated other comprehensive income	25	22
Cumulative employer contributions in excess of net periodic cost	(187)	(202)
Net (liability) asset recognized	(162)	(180)
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	6,802	6,354
Service cost	143	121	112
Interest cost	378	351	332
Employee contributions	14	16
Employer/Company contributions	320	237
Actuarial (gain) loss	575	430
Plan Amendments	(3)	0
Effect of Settlement	(7)	(17)
Effect of curtailment	(1)	(2)
Special termination benefits	0	1	0
Benefits paid	(266)	(244)
Foreign Currency Charges	66	(208)
Defined Benefit Plan Newly Disclosed Plans Obligation	16	0
Other	(2)	3
Benefit obligation at end of year	7,717	6,802	6,354
Fair value of plan assets at beginning of year	6,741	6,183
Actual return on plan assets	311	765
Effect of settlement	(6)	(17)
Foreign currency changes	82	(202)
Defined Benefit Plan Newly Disclosed Plans	12	0
Fair value of plan assets at end of year	7,206	6,741	6,183
Funded status	(511)	(61)
Noncurrent assets	39	265
Current liabilities	(106)	(8)
Noncurrent liabilities	(444)	(318)
Unrecognized prior service credit	23	23	24
Unrecognized net actuarial (loss) gain	(3,038)	(2,305)	(2,268)
Total amounts included in accumulated other comprehensive income	(3,015)	(2,282)
Cumulative employer contributions in excess of net periodic cost	2,504	2,221
Net (liability) asset recognized	(511)	(61)
Accumulated benefit obligation at December 31	7,246	6,313
Non- U.S Postretirement Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	83	70
Service cost	2	1	1
Interest cost	5	4	5
Employee contributions	0	0
Employer/Company contributions	4	3
Actuarial (gain) loss	5	11
Plan Amendments	0	0
Effect of Settlement	0	0
Effect of curtailment	0	0
Special termination benefits	0	0	0
Benefits paid	(4)	(3)
Foreign Currency Charges	0	0
Defined Benefit Plan Newly Disclosed Plans Obligation	0	0
Other	0	0
Benefit obligation at end of year	91	83	70
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Effect of settlement	0	0
Foreign currency changes	0	0
Defined Benefit Plan Newly Disclosed Plans	0	0
Fair value of plan assets at end of year	0	0	0
Funded status	(91)	(83)
Noncurrent assets	0	0
Current liabilities	(4)	(4)
Noncurrent liabilities	(87)	(79)
Unrecognized prior service credit	1	1	1
Unrecognized net actuarial (loss) gain	(19)	(14)	(4)
Total amounts included in accumulated other comprehensive income	(18)	(13)
Cumulative employer contributions in excess of net periodic cost	(73)	(70)
Net (liability) asset recognized	(91)	(83)
Accumulated benefit obligation at December 31	$ 0	$ 0