Acquisitions (Narrative) (Details) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2015 USD ($) acquisition	Jun. 30, 2014 USD ($)	Dec. 31, 2014 acquisition
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		8
Total Consideration		$ 331	$ 534
Cash		236	394
Estimated fair value of deferred/contingent consideration		95	140
Purchases of other intangible assets		3
Current Fiscal Period Acquisitions [Member]
Business Acquisition [Line Items]
Total Consideration		331
Cash		236
Estimated fair value of deferred/contingent consideration		95
Revenue related to acquisitions	$ 15	18
Operating income related to acquisitions	$ 0	1
Prior Fiscal Periods Acquisitions [Member]
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions		28	9
Contingent consideration from prior year's acquisitions		$ 33	$ 36
Risk and Insurance Services Segment [Member]
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition				15
Consulting Segment [Member]
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition				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
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