Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 2,030	$ 1,966	$ 4,316	$ 4,240
Interest and other income	[1]	18	24	44	42
Revenues		2,048	1,990	4,360	4,282
Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		955	899	2,096	2,034
Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	710	707	1,438	1,448
Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	234	247	518	520
Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	110	89	217	195
HCB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		807	792	1,644	1,634
Interest and other income	[1]	5	5	11	9
Revenues		812	797	1,655	1,643
HCB [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		64	61	137	137
HCB [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	571	558	1,151	1,135
HCB [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	101	116	224	248
HCB [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	56	40	103	83
CRB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		683	667	1,402	1,401
Interest and other income	[1]	7	7	16	13
Revenues		690	674	1,418	1,414
CRB [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		633	610	1,293	1,274
CRB [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	36	39	67	83
CRB [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	13	16	40	39
CRB [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	1	2	2	5
IRR [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		406	377	987	946
Interest and other income	[1]	5	9	15	16
Revenues		411	386	1,002	962
IRR [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		249	224	654	615
IRR [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	100	107	214	224
IRR [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	3		5
IRR [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	52	45	110	104
BDA [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		128	121	266	245
Revenues		128	121	266	245
BDA [Member] | Broking [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		9	4	12	8
BDA [Member] | Outsourced administration [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	117	115	249	233
Corporate, Non-Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		6	9	17	14
Interest and other income	[1]	1	3	2	4
Revenues		7	12	19	18
Corporate, Non-Segment [Member] | Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	3	3	6	6
Corporate, Non-Segment [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	1	2	2	3
Operating Segments [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		2,009	1,942	4,269	4,197
Revenues		2,022	1,958	4,303	4,226
Operating Segments [Member] | HCB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		792	775	1,615	1,603
Revenues		797	780	1,626	1,612
Operating Segments [Member] | CRB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		683	667	1,402	1,401
Revenues		690	674	1,418	1,414
Operating Segments [Member] | IRR [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		404	376	983	943
Revenues		409	385	998	959
Operating Segments [Member] | BDA [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		126	119	261	241
Revenues		126	119	261	241
Operating Segments [Member] | Corporate, Non-Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		4	5	8	9
Segment Reconciling Items [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]	21	24	47	43
Revenues		26	32	57	56
Segment Reconciling Items [Member] | HCB [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]	15	17	29	31
Segment Reconciling Items [Member] | IRR [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]	2	1	4	3
Segment Reconciling Items [Member] | BDA [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]	2	2	5	4
Segment Reconciling Items [Member] | Corporate, Non-Segment [Member]
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[3]	$ 2	$ 4	$ 9	$ 5

[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] Amounts presented for HCB Outsourced administration revenue include corrections of approximately $48 million and $106 million of revenue that was previously classified as HCB Consulting revenue or HCB Other revenue in our quarterly report on Form 10-Q for the three and six months ended June 30, 2018, respectively.

[3] 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.