Acquisitions / Dispositions (Narrative) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) acquisition	Dec. 31, 2014 USD ($) acquisition	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Total consideration		$ 1,200	$ 772
Cash		1,000	575
Estimated fair value of deferred/contingent consideration		176	197	$ 39
Deferred purchase consideration from prior years' acquisitions		36	25	15
Purchase of other intangibles		0	0	$ 2
Gain on disposition of business	$ 37
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration		1,180
Cash		1,004
Estimated fair value of deferred/contingent consideration		176
Revenue related to acquisitions		124
Net operating income related to acquisitions		7
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions		36
Deferred purchase consideration			25
Contingent payments for acquisitions		47	$ 42
Purchase of other intangibles		$ 2
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made | acquisition		13
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made | acquisition		8
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)		2 years	2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)		4 years	4 years
Marsh Insurance Group
Business Acquisition [Line Items]
Number of acquisitions made | acquisition			15
Mercer Consulting Group
Business Acquisition [Line Items]
Number of acquisitions made | acquisition			6