Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, numberOfColleague in Thousands, shares in Millions, $ in Millions, £ in Billions					1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 01, 2019 USD ($)	Apr. 01, 2019 USD ($) numberOfColleague $ / £	Apr. 01, 2019 GBP (£) numberOfColleague £ / shares $ / £	Sep. 20, 2018 GBP (£)	May 31, 2020 shares	Feb. 29, 2020 shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2020 USD ($) acquisition shares	Dec. 31, 2019 USD ($) acquisition shares	Dec. 31, 2018 USD ($)	Jan. 01, 2019
Business Acquisition [Line Items]
Total purchase consideration									$ 877	$ 5,927
Cash paid									694	5,861
Estimated contingent consideration									183	66	$ 133
Deferred purchase consideration									122	65	117
Revenue related to acquisitions									169	1,200	120
Net operating income (loss) related to acquisitions									11	(40)	2
Acquisition-related expenses incurred									3		7
Long-term indebtedness assumed									78	2,804	83
Cash payments for subsidiaries									$ 0	91	$ 0
Increase (decrease) in costs										$ 207
Risk and insurance services
Business Acquisition [Line Items]
Number of acquisitions made | acquisition									7	5
Certain Businesses Primarily in the U.S., U.K. and Canada
Business Acquisition [Line Items]
Cash proceeds from sale									$ 98
Alexander Forbes
Business Acquisition [Line Items]
Investment owned, balance (in shares) | shares									201
U.S. Specialty business and U.S. large market health and defined benefit business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Business Acquisition [Line Items]
Cash proceeds from sale							$ 60
JLT’s global aerospace business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Business Acquisition [Line Items]
Cash proceeds from sale	$ 165
Contingent consideration receivable	$ 65
Alexander Forbes
Business Acquisition [Line Items]
Equity investment shares owned by the Company (in shares) | shares								443		443
Sale of stock, number of shares issued in transaction (in shares) | shares					193	49
Marsh India
Business Acquisition [Line Items]
Percentage of interest acquired											26.00%	49.00%
Acquisitions made in prior years
Business Acquisition [Line Items]
Estimated contingent consideration									$ 102	$ 63
Deferred purchase consideration									$ 68	43
JLT Transaction
Business Acquisition [Line Items]
Total purchase consideration | £				£ 5.2
Acquisition-related expenses incurred										$ 125
Share price (in GBP per share) | £ / shares			£ 19.15
Consideration transferred, equity interests issued and issuable		$ 5,600	£ 4.3
Exchange rate (in USD per GBP) | $ / £		1.31	1.31
Long-term indebtedness assumed		$ 1,000
Number of colleagues | numberOfColleague		10	10
Cash payments for subsidiaries								$ 79
Minimum
Business Acquisition [Line Items]
Revenue target period									2 years
Maximum
Business Acquisition [Line Items]
Revenue target period									4 years