Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 1,974	$ 2,052	$ 4,124	$ 4,253
Reimbursable expenses and other	[1]	13	14	31	30
Interest and other income	[2]	57	39	67	66
Total revenue		2,031	2,091	4,191	4,319
HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,153	1,189	2,404	2,429
Reimbursable expenses and other	[1]	14	14	27	27
Interest and other income	[2]	20	4	26	10
Total revenue		1,173	1,193	2,430	2,439
R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		818	853	1,709	1,758
Reimbursable expenses and other	[1]	2	2	5	3
Interest and other income	[2]	36	34	39	54
Total revenue		854	887	1,748	1,812
Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	9	0	60
Reimbursable expenses and other	[1]	0	0	0	0
Interest and other income	[2]	0	0	0	0
Total revenue		0	9	0	60
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	3	1	11	6
Reimbursable expenses and other	[1]	(3)	(2)	(1)	(0)
Interest and other income	[1],[2]	1	1	2	2
Total revenue	[1]	4	2	13	8
Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		900	903	1,871	1,915
Broking [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		236	206	507	456
Broking [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		661	688	1,357	1,405
Broking [Member] | Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	9	0	54
Broking [Member] | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	3	0	7	0
Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		711	738	1,470	1,499
Consulting [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		617	637	1,270	1,289
Consulting [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		91	99	195	200
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]	3	2	5	4
Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		240	264	517	557
Outsourced administration [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		224	245	474	509
Outsourced administration [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		16	19	43	48
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		110	133	235	252
Other [Member] | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		62	87	126	148
Other [Member] | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		48	45	109	102
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	2
Service
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,961	2,038	4,093	4,223
Service | HWC [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,139	1,175	2,377	2,402
Service | R&B [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		816	851	1,704	1,755
Service | Divested Businesses [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0	9	0	60
Service | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	$ 6	$ 3	$ 12	$ 6

[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 and R&B’s interest and other income resulted primarily from book-of-business settlements. For HWC, these amounts totaled $15 million and $18 million, respectively, for three and six months ended June 30, 2022. For R&B, these amounts totaled $30 million for both the three and six months ended June 30, 2022. In the three and six months ended 2021, HWC had no significant settlements, and R&B had settlements totaling $31 million and $47 million, respectively.