Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, shares in Millions, $ in Millions, £ in Billions					1 Months Ended		3 Months Ended			6 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	Jun. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) acquisition shares	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2019 acquisition shares	Jan. 01, 2019
Business Acquisition [Line Items]
Total consideration										$ 737	$ 5,925
Cash paid										576	5,859
Estimated fair value of deferred/contingent consideration										161	66
Deferred purchase consideration from prior years' acquisitions										27	23
Contingent consideration from prior year's acquisitions										42	16
Assumption of debt										29	2,718
Payments to acquire outstanding minority interests of subsidiaries										0	91	$ 79
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions										27	23
Contingent consideration from prior year's acquisitions										78	45
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										737
Cash paid										576
Estimated fair value of deferred/contingent consideration										161
Revenue related to acquisitions							$ 48		$ 486	63	496
Operating (loss) income related to acquisitions							$ 7		$ 16	$ 11	$ 18
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												26.00%	26.00%	49.00%
Certain Businesses Primarily in the U.S., U.K. and Canada
Business Acquisition [Line Items]
Disposal group, consideration										$ 93
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
Alexander Forbes Group Holdings Limited
Business Acquisition [Line Items]
Investment owned, shares | shares							201			201
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