Acquisitions/Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions			1 Months Ended		12 Months Ended
	Sep. 18, 2018 USD ($) colleague	Sep. 18, 2018 GBP (£) colleague £ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($) acquisition	Dec. 31, 2017 USD ($) acquisition	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Total consideration					$ 1,040	$ 777
Cash						668
Estimated fair value of deferred/contingent consideration					133	109	$ 36
Deferred purchase consideration from prior years' acquisitions					62	55	54
Contingent payments for acquisitions					55	81	44
Deferred purchase consideration					117	136	98
Revenue related to acquisitions						156	25
Net operating income related to acquisitions						19	$ 4
Acquisition related costs					7	3
Gain on disposition of business			$ 46	$ 6
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration					1,043
Cash					910
Estimated fair value of deferred/contingent consideration					133
Revenue related to acquisitions					120
Net operating income related to acquisitions					2
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Estimated fair value of deferred/contingent consideration					91
Deferred purchase consideration from prior years' acquisitions						55
Contingent payments for acquisitions						$ 108
Deferred purchase consideration					$ 62
Jardine Lloyd Thompson Group plc
Business Acquisition [Line Items]
Total consideration | £		£ 5.2
Revenues	$ 2,000
Number of colleagues | colleague	10	10
Share price | £ / shares		£ 19.15
Consideration transferred, equity interests issued and issuable	$ 5,600	£ 4.3
Exchange rate	1.31	1.31
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made | acquisition					12	7
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made | acquisition					8	3
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)					2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)					4 years