Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015 acquisition	Mar. 31, 2014	Dec. 31, 2014 acquisition
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions)	3
Total Consideration	$ 32	$ 444
Cash	20	331
Estimated fair value of deferred/contingent consideration	12	113
Purchases of other intangible assets	3
Current Fiscal Period Acquisitions [Member]
Business Acquisition [Line Items]
Total Consideration	32
Cash	20
Estimated fair value of deferred/contingent consideration	12
Revenue related to acquisitions	3
Operating income related to acquisitions	0
Prior Fiscal Periods Acquisitions [Member]
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions	26	6
Contingent consideration from prior year's acquisitions	$ 19	$ 30
Risk and Insurance Services Segment [Member]
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions)			15
Consulting Segment [Member]
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions)			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