Acquisitions (Narrative) (Details) In Millions, except Per Share data, unless otherwise specified	6 Months Ended				3 Months Ended	6 Months Ended							12 Months Ended		0 Months Ended
	Jun. 30, 2014 USD ($) acquisition	Jun. 30, 2013 USD ($)	Jun. 30, 2014 Minimum [Member]	Jun. 30, 2014 Maximum [Member]	Jun. 30, 2014 Current Fiscal Period Acquisitions [Member] USD ($)	Jun. 30, 2014 Current Fiscal Period Acquisitions [Member] USD ($)	Jun. 30, 2013 Current Fiscal Period Acquisitions [Member] Minimum [Member]	Jun. 30, 2013 Current Fiscal Period Acquisitions [Member] Maximum [Member]	Jun. 30, 2014 Prior Fiscal Periods Acquisitions [Member] USD ($)	Jun. 30, 2013 Prior Fiscal Periods Acquisitions [Member] USD ($)	Jun. 23, 2014 Alexander Forbes Group Holdings Limited [Member] USD ($)	Jun. 23, 2014 Alexander Forbes Group Holdings Limited [Member] ZAR	Dec. 31, 2013 Risk and Insurance Services Segment [Member] acquisition	Dec. 31, 2013 Consulting Segment [Member] acquisition	Jul. 24, 2014 Subsequent Event [Member] Alexander Forbes Group Holdings Limited [Member] USD ($)	Jul. 24, 2014 Subsequent Event [Member] Alexander Forbes Group Holdings Limited [Member]	Oct. 01, 2014 Scenario, Forecast [Member] Alexander Forbes Group Holdings Limited [Member]
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions)	9												6	2
Total Consideration	$ 534	$ 109				$ 534									$ 140
Cash	394	95				394
Estimated fair value of deferred/contingent consideration	140	14				140
Revenue target period (in years)			2 years	4 years			2 years	4 years
Deferred purchase consideration from prior years' acquisitions	9	3							9	3
Contingent consideration from prior year's acquisitions									36	5
Revenue related to acquisitions					38	59
Operating income related to acquisitions					5	6
Business Acquisition, Percentage of Interests Acquired, Planned Acquisition											34.00%	34.00%
Business Acquisition, Consideration to be Transferred, Planned Acquisition											$ 312
Business Acquisition, Share Price, Planned Acquisition												7.50
Business Acquisition, Percentage of Voting Interests Acquired																14.90%	19.10%