Acquisitions (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended				12 Months Ended
	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Feb. 29, 2012 Acquisition Yokogawa-ORC [Member]	Dec. 31, 2013 Current Fiscal Period Acquisitions [Member]	Dec. 31, 2013 Prior Fiscal Periods Acquisitions [Member]	Dec. 31, 2012 Prior Fiscal Periods Acquisitions [Member]	Dec. 31, 2013 Risk and Insurance Services Segment [Member] acquisition	Dec. 31, 2013 Consulting Segment [Member] acquisition	Dec. 31, 2013 Maximum [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2013 Minimum [Member]	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Marsh Insurance Group [Member] acquisition	Dec. 31, 2012 Mercer Consulting Group [Member] acquisition
Business Acquisition [Line Items]
Number of acquisitions made									6	2					12	3
Total Consideration		$ 178	$ 360			$ 178
Cash		139	252			139
Estimated fair value of deferred/contingent consideration		39	46	33		39
Revenue target period (in years)											4 years	4 years	2 years	2 years
Deferred purchase consideration from prior years' acquisitions		15	59	11			15
Contingent payments for acquisitions							17	30
Purchase of other intangibles		2	3	4
Business combination remaining interest acquired					49.00%
Cash held in escrow related to acquisition	62	0	62	0
Deferred purchase consideration								59
Revenue related to acquisitions						50
Net operating income related to acquisitions						$ 9