Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 18, 2018 USD ($) colleague	Sep. 18, 2018 GBP (£) colleague £ / shares	Jul. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) acquisition	Sep. 30, 2017 USD ($)	Dec. 31, 2017 acquisition
Business Acquisition [Line Items]
Total consideration						$ 661	$ 734
Cash						553	640
Estimated fair value of deferred/contingent consideration						108	94
Contingent consideration from prior year's acquisitions						47	80
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration						661
Cash						553
Estimated fair value of deferred/contingent consideration						108
Revenue related to acquisitions				$ 36	$ 50	40	103
Operating (loss) income related to acquisitions				$ (5)	$ (7)	(1)	16
Jardine Lloyd Thompson Group plc
Business Acquisition [Line Items]
Total consideration	$ 5,200
Revenue	2,000
Equity interests issued and issuable	$ 5,600	£ 4.3
Business combination, exchange rate	1.31	1.31
Threshold percentage value of majority shareholders	75.00%	75.00%
Number of colleagues | colleague	10	10
Business acquisition, share price | £ / shares		£ 19.15
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions						59	47
Contingent consideration from prior year's acquisitions						$ 76	$ 107
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition						8		7
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition						5		3
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)						2 years
Minimum | Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Revenue target period (in years)						2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)						4 years
Maximum | Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Revenue target period (in years)						4 years
Marsh ClearSight
Business Acquisition [Line Items]
Gain on disposition of business			$ 46