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	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2010	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2012	Feb. 29, 2012 Acquisition Yokogawa-ORC [Member]	Sep. 30, 2012 Business Acquisition [Member]	Sep. 30, 2012 Business Acquisition [Member]	Sep. 30, 2012 Purchase Consideration for Acquisitions During the Period [Member]	Sep. 30, 2011 Purchase Consideration for Acquisitions During the Period [Member]	Sep. 30, 2012 Cash [Member]	Sep. 30, 2012 Released Cash Held in Escrow [Member]	Sep. 30, 2011 Prior Years Acquisitions [Member]	Sep. 30, 2011 Minority Interest Acquired [Member]	Sep. 30, 2012 Risk and Insurance Services [Member] Acquisitions	Dec. 31, 2011 Risk and Insurance Services [Member] Acquisitions	Sep. 30, 2012 Consulting [Member] Acquisitions	Dec. 31, 2011 Consulting [Member] Acquisitions
Number of acquisitions made (in acquisitions)															7	7	3	5
Business combination remaining percent acquired						49.00%
Total Consideration	$ 205		$ 205	$ 132					$ 205					$ 8
Adjustments to Additional Paid in Capital, Other														2
Cash	186		186							116	124	62
Contingent consideration	19		19	16						16
Revenue target period, in years			two to four years
Revenue target period minimum (in years)			2 years
Revenue target period maximum (in years)			4 years
Deferred purchase consideration									51	13			19
Other intangible assets	96		96	2
Contingent payments for acquisitions			(20)						20
Revenue related to acquisitions	2,848	2,850	8,946	8,762			31	76
Net operating income related to acquisitions	247	133	943	738			6	15
Subsequent event amount					$ 60