Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended		3 Months Ended	9 Months Ended				3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Mar. 31, 2010	Mar. 31, 2010 Risk and Insurance Services [Member]	Sep. 30, 2011 Risk and Insurance Services [Member]	Sep. 30, 2010 Risk and Insurance Services [Member]	Sep. 30, 2011 Consulting [Member]	Sep. 30, 2010 Consulting [Member]	Sep. 30, 2011 Business Acquisition [Member]	Sep. 30, 2011 Business Acquisition [Member]	Sep. 30, 2010 Purchase Consideration for Acquisitions During the Period [Member]	Jun. 30, 2011 Purchase Consideration for Acquisitions During the Period [Member]	Mar. 31, 2011 Purchase Consideration for Acquisitions During the Period [Member]	Sep. 30, 2011 Prior Years Acquisitions [Member]	Dec. 31, 2010 Prior Years Acquisitions [Member]
Cash	$ 116		$ 116											$ 282
Contingent consideration	16	65	16	65										65				2
Total Consideration	132	530	132	530											8	132
Adjustments to Additional Paid in Capital, Other			2
Deferred purchase consideration						15										13	19	21
Purchase of other intangible assets			2	3	3
Number of acquisitions made				6			4	4	4	3	2
Issuance of shares														7.6
Fair value														183
Revenue target period, in years			two to four years	two to four years
Revenue related to acquisitions	2,806	2,568	8,630	7,985								21	55
Net operating income related to acquisitions	$ 133	$ 131	$ 741	$ 383								$ 2	$ 8