Acquisitions (Narrative) (Details) In Millions, except Per Share data, unless otherwise specified	9 Months Ended		3 Months Ended	9 Months Ended			12 Months Ended		9 Months Ended							0 Months Ended	1 Months Ended
	Sep. 30, 2014 USD ($) acquisition	Sep. 30, 2013 USD ($)	Sep. 30, 2014 Current Fiscal Period Acquisitions [Member] USD ($)	Sep. 30, 2014 Current Fiscal Period Acquisitions [Member] USD ($)	Sep. 30, 2014 Prior Fiscal Periods Acquisitions [Member] USD ($)	Sep. 30, 2013 Prior Fiscal Periods Acquisitions [Member] USD ($)	Dec. 31, 2013 Risk and Insurance Services Segment [Member] acquisition	Dec. 31, 2013 Consulting Segment [Member] acquisition	Sep. 30, 2014 Minimum [Member]	Sep. 30, 2013 Minimum [Member] Current Fiscal Period Acquisitions [Member]	Sep. 30, 2014 Maximum [Member]	Sep. 30, 2013 Maximum [Member] Current Fiscal Period Acquisitions [Member]	Jul. 24, 2014 Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member] ZAR	Jun. 23, 2014 Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member]	Oct. 01, 2014 Mercer Consulting Group [Member] Alexander Forbes Group Holdings Limited [Member] Subsequent Event [Member]	Jul. 24, 2014 Mercer Consulting Group [Member] Other Assets [Member] Alexander Forbes Group Holdings Limited [Member] USD ($)	Oct. 31, 2014 Mercer Consulting Group [Member] Other Assets [Member] Alexander Forbes Group Holdings Limited [Member] Subsequent Event [Member] USD ($)
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions)	12						6	2
Total Consideration	$ 573	$ 156		$ 573												$ 137	$ 166
Cash	426	119		426
Estimated fair value of deferred/contingent consideration	147	37		147
Revenue target period (in years)									2 years	2 years	4 years	4 years
Deferred purchase consideration from prior years' acquisitions	10	4			10	4
Contingent consideration from prior year's acquisitions					40	8
Revenue related to acquisitions			46	104
Operating income related to acquisitions			$ 5	$ 11
Business Acquisition, Percentage of Interests Acquired, Planned Acquisition														34.00%
Business Acquisition, Share Price													7.50
Business Acquisition, Percentage of Voting Interests Acquired													14.90%		19.10%