Segment Information (Details of Operating Segment Revenue) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended				9 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended				9 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Operating Segments [Member]	Sep. 30, 2012 Operating Segments [Member]	Sep. 30, 2013 Operating Segments [Member]	Sep. 30, 2012 Operating Segments [Member]	Sep. 30, 2013 Operating Segments [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Operating Segments [Member] Marsh Insurance Group [Member]	Sep. 30, 2013 Operating Segments [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Operating Segments [Member] Marsh Insurance Group [Member]	Sep. 30, 2013 Operating Segments [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2012 Operating Segments [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2013 Operating Segments [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2012 Operating Segments [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2013 Operating Segments [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Operating Segments [Member] Mercer Consulting Group [Member]	Sep. 30, 2013 Operating Segments [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Operating Segments [Member] Mercer Consulting Group [Member]	Sep. 30, 2013 Operating Segments [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2012 Operating Segments [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2013 Operating Segments [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2012 Operating Segments [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2013 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2013 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2012 Operating Segments [Member] Risk and Insurance Services Segment [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2013 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2013 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member]		Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2012 Operating Segments [Member] Consulting Segment [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2013 Intersegment Eliminations [Member]	Sep. 30, 2012 Intersegment Eliminations [Member]	Sep. 30, 2013 Intersegment Eliminations [Member]	Sep. 30, 2012 Intersegment Eliminations [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Dec. 31, 2012 Intersegment Eliminations [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2012 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2013 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2012 Intersegment Eliminations [Member] Consulting Segment [Member]	Sep. 30, 2013 Corporate, Non-Segment [Member]	Sep. 30, 2012 Corporate, Non-Segment [Member]	Sep. 30, 2013 Corporate, Non-Segment [Member]	Sep. 30, 2012 Corporate, Non-Segment [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Consulting Segment [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Consulting Segment [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Consulting Segment [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Consulting Segment [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Consulting Segment [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Operating Segments [Member] Consulting Segment [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Intersegment Eliminations [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Intersegment Eliminations [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Corporate, Non-Segment [Member]	Sep. 30, 2012 Scenario, Previously Reported [Member] Corporate, Non-Segment [Member]	Sep. 30, 2012 Restatement Adjustment [Member]	Sep. 30, 2012 Restatement Adjustment [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Marsh Insurance Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Risk and Insurance Services Segment [Member] Guy Carpenter Reinsurance Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Consulting Segment [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Consulting Segment [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Consulting Segment [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Consulting Segment [Member] Mercer Consulting Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Consulting Segment [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Operating Segments [Member] Consulting Segment [Member] Oliver Wyman Group Consulting Group [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Intersegment Eliminations [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Intersegment Eliminations [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Corporate, Non-Segment [Member]	Sep. 30, 2012 Restatement Adjustment [Member] Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenue	$ 2,932	$ 2,845	$ 9,146	$ 8,922	$ 2,941	$ 2,856	$ 9,172	$ 8,955	$ 1,241	$ 1,201	$ 4,038	$ 3,895	$ 263	$ 250	$ 925	$ 886	$ 1,072	$ 1,054	$ 3,157	$ 3,086	$ 365	$ 351	$ 1,052	$ 1,088	$ 1,504	[1]	$ 1,451	[1]	$ 4,963	[2]	$ 4,781	[2]	$ 1,201	$ 3,895	$ 250	$ 886	$ 1,437	[3]	$ 1,405	[3]	$ 4,209	[4]	$ 4,174	[4]	$ 1,054	$ 3,086	$ 351	$ 1,088	$ (9)	$ (11)	$ (26)	$ (33)	$ 1	$ 1	$ 4	$ 4	$ 8	$ 10	$ 22	$ 29					$ 2,845	$ 8,922	$ 2,856	$ 8,955	$ 1,510	$ 4,955	$ 1,260	$ 4,069	$ 250	$ 886	$ 1,346	$ 4,000	$ 995	$ 2,912	$ 351	$ 1,088	$ (11)	$ (33)			$ 0	$ 0	$ 0	$ 0	$ (59)	$ (174)	$ (59)	$ (174)	$ 0	$ 0	$ 59	$ 174	$ 59	$ 174	$ 0	$ 0	$ 0	$ 0
Operating Income (Loss)	$ 404	$ 378	$ 1,588	$ 1,423	$ 454	$ 427	$ 1,735	$ 1,576																	$ 222		$ 222		$ 1,111		$ 1,024						$ 232		$ 205		$ 624		$ 552																		$ (50)	$ (49)	$ (147)	$ (153)	$ 378	$ 1,423	$ 427	$ 1,576	$ 234	$ 1,052					$ 193	$ 524							$ (49)	$ (153)	$ 0	$ 0	$ 0	$ 0	$ (12)	$ (28)					$ 12	$ 28							$ 0	$ 0

[1] Includes inter-segment revenue of $1 million in both 2013 and 2012, interest income on fiduciary funds of $7 million and $10 million in 2013 and 2012, respectively, and equity method income of $2 million in 2013.

[2] Includes inter-segment revenue of $4 million in both 2013 and 2012, interest income on fiduciary funds of $21 million and $31 million in 2013 and 2012, respectively, and equity method income of $10 million in both 2013 and 2012.

[3] Includes inter-segment revenue of $8 million and $10 million in 2013 and 2012, respectively, and interest income on fiduciary funds of $1 million in both 2013 and 2012.

[4] Includes inter-segment revenue of $22 million and $29 million in 2013 and 2012, respectively, and interest income on fiduciary funds of $3 million and $2 million in 2013 and 2012, respectively.