Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions					3 Months Ended		6 Months Ended
	Jun. 01, 2019 USD ($)	Apr. 01, 2019 USD ($)	Apr. 01, 2019 GBP (£) colleague £ / shares	Sep. 20, 2018 GBP (£)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) acquisition	Jun. 30, 2018 USD ($) acquisition	Jan. 01, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Assumption of debt							$ 2,718	$ 28
Total consideration							5,925	192
Cash							5,859	160
Estimated fair value of deferred/contingent consideration							66	32
Deferred purchase consideration from prior years' acquisitions								53
Contingent consideration from prior year's acquisitions							16	32
Increase (decrease) in costs					$ (151)		(169)	658
Jardine Lloyd Thompson Group plc
Business Acquisition [Line Items]
Business acquisition, share price (in usd 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
Business combination, acquisition related costs					84		95
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					486	$ 11	496	14
Operating (loss) income related to acquisitions					$ 16	$ (1)	18	(2)
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions							23	53
Contingent consideration from prior year's acquisitions							$ 45	$ 41
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
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 India
Business Acquisition [Line Items]
Percentage of ownership in equity investment									49.00%	26.00%
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