Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions					3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Jun. 01, 2019 USD ($)	Apr. 01, 2019 USD ($)	Apr. 01, 2019 GBP (£) colleague £ / shares	Sep. 20, 2018 GBP (£)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) acquisition	Sep. 30, 2018 USD ($)	Dec. 31, 2018 acquisition	Jan. 01, 2019
Business Acquisition [Line Items]
Assumption of debt								$ 2,758	$ 35
Total consideration								5,925	661
Cash								5,859	553
Estimated fair value of deferred/contingent consideration								66	108
Deferred purchase consideration from prior years' acquisitions								37	59
Contingent consideration from prior year's acquisitions								23	47
Amortization of intangible assets								235	135
Payments to acquire outstanding minority interests of subsidiaries								91	0
Increase (decrease) in costs					$ (4)			(173)	662
Jardine Lloyd Thompson Group plc
Business Acquisition [Line Items]
Business acquisition, share price (in gbp per share) | £ / shares			£ 19.15
Equity interests issued and issuable		$ 5,600	£ 4.3
Assumption of debt		$ 1,000
Entity number of employees | colleague			10
Total consideration				£ 5.2				5,568
Cash								5,568
Estimated fair value of deferred/contingent consideration								0
Adjustments to intangible assets					(50)
Goodwill, purchase accounting adjustments					65
Business combination, acquisition related costs					4			99
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions								37	59
Contingent consideration from prior year's acquisitions								58	76
Payments to acquire outstanding minority interests of subsidiaries							$ 75
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration								5,925
Cash								5,859
Estimated fair value of deferred/contingent consideration								66
Revenue related to acquisitions					352	$ 36		848	50
Operating (loss) income related to acquisitions					(69)	$ (5)		$ (51)	$ (7)
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition								4		12
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition										8
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)								2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)								4 years
Marsh India
Business Acquisition [Line Items]
Percentage of ownership in equity investment										26.00%	49.00%
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
Adjustment | Jardine Lloyd Thompson Group plc
Business Acquisition [Line Items]
Amortization of intangible assets					$ (4)