Acquisitions / Dispositions (Narrative) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) acquisition	Dec. 31, 2015 USD ($) acquisition	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Total consideration		$ 901	$ 1,200
Cash			1,000
Estimated fair value of deferred/contingent consideration		36	176	$ 197
Deferred purchase consideration from prior years' acquisitions		54	36	25
Contingent payments for acquisitions		44	13	30
Deferred purchase consideration		98	49	$ 55
Revenue related to acquisitions			124
Net operating income related to acquisitions			7
Acquisition related expenses		14
Gain on disposition of business	$ 37	6
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration		901
Cash		865
Estimated fair value of deferred/contingent consideration		36
Revenue related to acquisitions		25
Net operating income related to acquisitions		4
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions		54
Contingent payments for acquisitions		$ 86	47
Deferred purchase consideration			36
Purchase of other intangible assets			$ 2
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made | acquisition		9
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made | acquisition		6
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			13
Mercer Consulting Group
Business Acquisition [Line Items]
Number of acquisitions made | acquisition			8