Acquisitions (Narrative) (Details) £ in Millions, $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) acquisition	Sep. 30, 2014 USD ($)	Dec. 31, 2014 acquisition	Sep. 10, 2015 USD ($)	Sep. 10, 2015 GBP (£)
Business Acquisition [Line Items]
Total consideration		$ 602	$ 573
Cash		448	426
Estimated fair value of deferred/contingent consideration		154	147
Purchases of other intangible assets		2
Current Fiscal Period Acquisitions [Member]
Business Acquisition [Line Items]
Total consideration		602
Cash		448
Estimated fair value of deferred/contingent consideration		154
Revenue related to acquisitions	$ 48	66
Operating income related to acquisitions	$ 3	4
Prior Fiscal Periods Acquisitions [Member]
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions		30	10
Contingent consideration from prior year's acquisitions		$ 39	$ 40
Jelf Group PLC [Member]
Business Acquisition [Line Items]
Percentage of interest to be acquired					100.00%	100.00%
Acquisition consideration to be transferred					$ 400	£ 258
Risk and Insurance Services Segment [Member]
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		11		15
Consulting Segment [Member]
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		5		6
Minimum [Member]
Business Acquisition [Line Items]
Revenue target period (in years)		2 years
Minimum [Member] | Current Fiscal Period Acquisitions [Member]
Business Acquisition [Line Items]
Revenue target period (in years)		2 years	2 years
Maximum [Member]
Business Acquisition [Line Items]
Revenue target period (in years)		4 years
Maximum [Member] | Current Fiscal Period Acquisitions [Member]
Business Acquisition [Line Items]
Revenue target period (in years)		4 years	4 years