Acquisitions (Narrative) (Details) In Millions, except Per Share data, unless otherwise specified	12 Months Ended																		0 Months Ended	1 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 Current Fiscal Period Acquisitions [Member] USD ($)	Dec. 31, 2014 Prior Fiscal Periods Acquisitions [Member] USD ($)	Dec. 31, 2013 Prior Fiscal Periods Acquisitions [Member] USD ($)	Dec. 31, 2014 Risk and Insurance Services Segment [Member] acquisition	Dec. 31, 2014 Consulting Segment [Member] acquisition	Dec. 31, 2014 Maximum [Member]	Dec. 31, 2013 Maximum [Member]	Dec. 31, 2014 Minimum [Member]	Dec. 31, 2013 Minimum [Member]	Dec. 31, 2013 Marsh Insurance Group [Member] acquisition	Dec. 31, 2013 Mercer Consulting Group [Member] acquisition	Dec. 31, 2014 Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member] USD ($)	Oct. 01, 2014 Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member]	Jul. 24, 2014 Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member] ZAR	Jun. 30, 2014 Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member]	Jul. 24, 2014 Other Assets [Member] Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member] USD ($)	Oct. 31, 2014 Other Assets [Member] Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member] USD ($)
Business Acquisition [Line Items]
Number of acquisitions made							15	6					6	2
Total Consideration	$ 772	$ 178		$ 772															$ 137	$ 166
Cash	575	139		575
Estimated fair value of deferred/contingent consideration	197	39	46	197
Revenue target period (in years)									4 years	4 years	2 years	2 years
Deferred purchase consideration from prior years' acquisitions	25	15	59		25
Deferred purchase consideration						15
Contingent payments for acquisitions					42	17
Purchase of other intangibles	0	2	3
Revenue related to acquisitions				134
Net operating income related to acquisitions				18
Percentage of interest acquired																		34.00%
Share price																	7.50
Business combination remaining interest acquired																19.10%	14.90%
Value of carrying amount that exceeds share in equity method investment															$ 146