Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2010	Dec. 31, 2011 Current Year Acquisitions [Member]	Dec. 31, 2011 Minority Interest Acquired [Member]	Jun. 30, 2011 Minority Interest Acquired [Member]	Dec. 31, 2010 Prior Years Acquisitions [Member]	Mar. 31, 2010 Prior Years Acquisitions [Member]	Dec. 31, 2010 Remaining Prior Years Acquisitions [Member]	Dec. 31, 2011 Risk and Insurance Services [Member]	Dec. 31, 2010 Risk and Insurance Services [Member]	Dec. 31, 2011 Consulting [Member]	Dec. 31, 2010 Consulting [Member]
Cash					$ 164			$ 490
Deferred/Contingent consideration	33	81	100		33			81		2
Total Consideration	197				197		8	754
Adjustments to Additional Paid in Capital, Other						2
Deferred purchase consideration				15	27				13	60
Purchase of other intangible assets	4	3	0		4
Number of acquisitions made		8									7	6	5	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	11,563	10,839	10,436		79
Net operating income related to acquisitions	$ 985	$ 580	$ 572		$ 12