Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 1,928	$ 1,952	$ 6,052	$ 6,205
Reimbursable expenses and other	[1]	22	5	53	35
Interest and other income	[2]	25	21	92	87
Total revenue		1,953	1,973	6,144	6,292
HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,172	1,177	3,576	3,606
Reimbursable expenses and other	[1]	16	13	43	40
Interest and other income	[2]	6	4	32	14
Total revenue		1,178	1,181	3,608	3,620
R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		749	774	2,458	2,532
Reimbursable expenses and other	[1]	2	2	7	5
Interest and other income	[2]	18	15	57	69
Total revenue		767	789	2,515	2,601
Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	8	0	68
Reimbursable expenses and other	[1]	0	0	0	0
Interest and other income	[2]	0	0	0	0
Total revenue	[3]	0	8	0	68
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	7	(7)	18	(1)
Reimbursable expenses and other	[1]	4	(10)	3	(10)
Interest and other income	[1],[2]	1	2	3	4
Total revenue	[1]	8	(5)	21	3
Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		814	818	2,685	2,733
Broking [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		207	189	714	645
Broking [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		606	621	1,963	2,026
Broking [Member] | Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	8	0	62
Broking [Member] | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	1	0	8	0
Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		703	715	2,173	2,214
Consulting [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		616	621	1,886	1,910
Consulting [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		85	92	280	292
Consulting [Member] | Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	0	0	6
Consulting [Member] | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	2	2	7	6
Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		240	266	757	823
Outsourced administration [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		222	245	696	754
Outsourced administration [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		18	21	61	69
Outsourced administration [Member] | Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	0	0	0
Outsourced administration [Member] | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	0	0	0	0
Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		149	148	384	400
Other [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		111	109	237	257
Other [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		38	38	147	140
Other [Member] | Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	0	0	0
Other [Member] | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	0	1	0	3
Service
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,906	1,947	5,999	6,170
Service | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,156	1,164	3,533	3,566
Service | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		747	772	2,451	2,527
Service | Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	8	0	68
Service | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	$ 3	$ 3	$ 15	$ 9

[1] Reimbursable expenses and other, as well as Corporate revenue, are excluded from segment revenue, but included in total revenue on the condensed consolidated statements of comprehensive income. Amounts included in Corporate revenue may include eliminations and impacts from hedged revenue transactions.

[2] Interest and other income is included in segment revenue and total revenue, however it has been presented separately in the above tables because it does not arise directly from contracts with customers. In 2022, both HWC’s and R&B’s interest and other income resulted primarily from book-of-business settlements. For HWC, these amounts totaled $2 million and $19 million, respectively, for the three and nine months ended September 30, 2022. For R&B, these amounts totaled $11 million and $41 million, respectively, for the three and nine months ended September 30, 2022. In the three and nine months ended September 30, 2021, HWC had no material settlements, and R&B had settlements totaling $12 million and $59 million, respectively.

[3] Represents the revenue and income from operations of certain Investment, Risk and Reinsurance businesses which were divested in 2021 and not classified as discontinued operations.