Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, shares in Millions, $ in Millions, £ in Billions					1 Months Ended		3 Months Ended		9 Months Ended			12 Months Ended
	Jun. 01, 2019 USD ($)	Apr. 01, 2019 USD ($) $ / £	Apr. 01, 2019 GBP (£) £ / shares $ / £	Sep. 20, 2018 GBP (£)	May 31, 2020 shares	Feb. 29, 2020 shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) acquisition shares	Dec. 31, 2019 USD ($) shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 acquisition shares	Jan. 01, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Total consideration									$ 742		$ 5,925
Cash paid									579		5,859
Estimated fair value of deferred/contingent consideration									163		66
Deferred purchase consideration from prior years' acquisitions									60		37
Contingent consideration from prior year's acquisitions									65		23
Disposal group, consideration							$ 0
Assumption of debt									73		2,758
Payments to acquire outstanding minority interests of subsidiaries									0	$ 75	91
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions									60		37
Contingent consideration from prior year's acquisitions									101		58
Jardine Lloyd Thompson Group plc
Business Acquisition [Line Items]
Total consideration | £				£ 5.2
Business acquisition, share price (in gbp per share) | £ / shares			£ 19.15
Equity interests issued and issuable		$ 5,600	£ 4.3
Business combination, exchange rate | $ / £		1.31	1.31
Assumption of debt		$ 1,000
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration									742
Cash paid									579
Estimated fair value of deferred/contingent consideration									163
Revenue related to acquisitions							52	$ 352	115		848
Operating (loss) income related to acquisitions							$ (8)	(69)	$ 3		$ (51)
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition									4			5
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)									2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)									4 years
Alexander Forbes Group Holdings Limited
Business Acquisition [Line Items]
Equity method investment, shares owned | shares										443		443
Number of shares sold (shares) | shares					193	49
Marsh India
Business Acquisition [Line Items]
Percentage of ownership in equity investment													49.00%	26.00%
Certain Businesses Primarily in the U.S., U.K. and Canada
Business Acquisition [Line Items]
Disposal group, consideration									$ 93
Alexander Forbes Group Holdings Limited
Business Acquisition [Line Items]
Investment owned, shares | shares							201		201
Specialty Business and Large Market Health and Defined Benefit Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Business Acquisition [Line Items]
Disposal group, consideration								$ 60
JLT’s Global Aerospace Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Business Acquisition [Line Items]
Disposal group, consideration	$ 165
Contingent consideration receivable	$ 65