Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2014 acquisition	Mar. 31, 2013	Mar. 31, 2014 Minimum [Member]	Mar. 31, 2014 Maximum [Member]	Mar. 31, 2014 Current Fiscal Period Acquisitions [Member]	Mar. 31, 2014 Current Fiscal Period Acquisitions [Member] Minimum [Member]	Mar. 31, 2014 Current Fiscal Period Acquisitions [Member] Maximum [Member]	Mar. 31, 2014 Prior Fiscal Periods Acquisitions [Member]	Dec. 31, 2013 Risk and Insurance Services Segment [Member] acquisition	Dec. 31, 2013 Consulting Segment [Member] acquisition
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions)	6								6	2
Total Consideration	$ 444				$ 444
Cash	331				331
Estimated fair value of deferred/contingent consideration	113	0			113
Revenue target period (in years)			2 years	4 years		2 years	4 years
Deferred purchase consideration from prior years' acquisitions	6	1						6
Contingent consideration from prior year's acquisitions								30
Revenue related to acquisitions					21
Operating income related to acquisitions					$ 1