Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, $ in Millions, £ in Billions					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 (£)	Mar. 31, 2020 USD ($) acquisition	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 acquisition	Jun. 30, 2020	Feb. 29, 2020	Jan. 01, 2019
Business Acquisition [Line Items]
Total consideration					$ 245		$ 177
Cash paid					201		142
Estimated fair value of deferred/contingent consideration					44		35
Deferred purchase consideration from prior years' acquisitions					25		23
Contingent consideration from prior year's acquisitions					4		6
Assumption of debt					5		5
Payments to acquire outstanding minority interests of subsidiaries					0		88	$ 79
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions					25		23
Contingent consideration from prior year's acquisitions					13		35
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					245
Cash paid					201
Estimated fair value of deferred/contingent consideration					44
Revenue related to acquisitions					15		4
Operating (loss) income related to acquisitions					$ 4		$ 1
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition					2				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]
Percentage of equity method investment sold											4.00%
Percentage of ownership in equity investment					31.00%
Marsh India
Business Acquisition [Line Items]
Percentage of ownership in equity investment								26.00%	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
Forecast | Alexander Forbes Group Holdings Limited
Business Acquisition [Line Items]
Percentage of equity method investment sold										15.00%