Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, colleague in Thousands, $ in Millions, £ in Billions					1 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended
	Jun. 01, 2019 USD ($)	Apr. 01, 2019 USD ($) colleague $ / £	Apr. 01, 2019 GBP (£) colleague £ / shares $ / £	Sep. 20, 2018 GBP (£)	Jan. 31, 2020 insurance_broker	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) acquisition	Dec. 31, 2018 USD ($) acquisition	Dec. 31, 2017 USD ($)	Jan. 01, 2019	Dec. 31, 2014 R / shares
Business Acquisition [Line Items]
Long-term indebtedness assumed										$ 2,804	$ 83	$ 134
Total purchase consideration										5,927	1,040
Cash paid											910
Estimated contingent consideration										66	133	109
Deferred purchase consideration										65	117	136
Cash payments for subsidiaries										91	0	0
Increase (decrease) in costs											207
Revenue related to acquisitions										1,200	120	156
Net operating income (loss) related to acquisitions										(40)	2	$ 19
Acquisition-related expenses incurred											7
Pre-tax gain on sale						$ 46
JLT Transaction
Business Acquisition [Line Items]
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 | colleague		10	10
Total purchase consideration				£ 5.2						5,568
Cash paid										5,568
Estimated contingent consideration										0
Increase to goodwill							$ 90
Cash payments for subsidiaries									$ 79
Acquisition-related expenses incurred										125
Acquisitions made during 2019
Business Acquisition [Line Items]
Cash paid										5,861
Estimated contingent consideration										66
Acquisitions made in prior years
Business Acquisition [Line Items]
Estimated contingent consideration										63	91
Deferred purchase consideration										$ 43	$ 62
Risk and insurance services
Business Acquisition [Line Items]
Number of acquisitions made | acquisition										5	12
Consulting segment
Business Acquisition [Line Items]
Number of acquisitions made | acquisition											8
Minimum
Business Acquisition [Line Items]
Revenue target period										2 years
Maximum
Business Acquisition [Line Items]
Revenue target period										4 years
Marsh India
Business Acquisition [Line Items]
Percentage of interest acquired											26.00%		49.00%
Alexander Forbes
Business Acquisition [Line Items]
Share price (in GBP per share) | R / shares														R 7.50
Percentage of interest acquired							34.00%		34.00%	34.00%				34.00%
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
Subsequent Event | Momentos Insurance Brokerage and Ironwood Insurance Services
Business Acquisition [Line Items]
Number of independent insurance brokers acquired (insurance broker) | insurance_broker					2