Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 2,101	$ 2,030	$ 4,535	$ 4,316
Interest and other income	[1]	12	18	44	44
Revenues		2,113	2,048	4,579	4,360
Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,056	955	2,318	2,096
Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		668	710	1,392	1,438
Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		260	234	549	518
Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		98	110	230	217
HCB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		776	807	1,631	1,644
Interest and other income	[1]	2	5	12	11
Revenues		778	812	1,643	1,655
HCB [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		71	64	154	137
HCB [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		531	571	1,113	1,151
HCB [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		122	101	250	224
HCB [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		41	56	88	103
CRB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		695	683	1,416	1,402
Interest and other income	[1]	7	7	25	16
Revenues		702	690	1,441	1,418
CRB [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		637	633	1,285	1,293
CRB [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		40	36	87	67
CRB [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		16	13	41	40
CRB [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1	1	2	2
IRR [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		412	406	1,027	987
Interest and other income	[1]	2	5	5	15
Revenues		414	411	1,032	1,002
IRR [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		258	249	691	654
IRR [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		95	100	188	214
IRR [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		3	3	6	5
IRR [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		55	52	138	110
BDA [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		211	128	445	266
Revenues		211	128	445	266
BDA [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		90	9	188	12
BDA [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		119	117	252	249
Corporate, Non-Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	7	6	16	17
Interest and other income	[1],[2]	1	1	2	2
Revenues	[2]	8	7	18	19
Corporate, Non-Segment [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	2	3	4	6
Corporate, Non-Segment [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	1	1	2	2
Operating Segments [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		2,082	2,009	4,489	4,269
Revenues		2,090	2,022	4,525	4,303
Operating Segments [Member] | HCB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		765	792	1,605	1,615
Revenues		767	797	1,617	1,626
Operating Segments [Member] | CRB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		694	683	1,415	1,402
Revenues		701	690	1,440	1,418
Operating Segments [Member] | IRR [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		411	404	1,023	983
Revenues		413	409	1,028	998
Operating Segments [Member] | BDA [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		209	126	440	261
Revenues		209	126	440	261
Operating Segments [Member] | Corporate, Non-Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	3	4	6	8
Segment Reconciling Items [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	19	21	46	47
Revenues		23	26	54	57
Segment Reconciling Items [Member] | HCB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	11	15	26	29
Segment Reconciling Items [Member] | CRB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	1		1
Segment Reconciling Items [Member] | IRR [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	1	2	4	4
Segment Reconciling Items [Member] | BDA [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	2	2	5	5
Segment Reconciling Items [Member] | Corporate, Non-Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	$ 4	$ 2	$ 10	$ 9

[1] 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.

[2] 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.