Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		$ 1,826		$ 6,066
Revenues		1,859	$ 1,852	6,141	$ 6,124
Broking [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		778		2,812
Consulting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		718		2,264
Outsourced administration [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		207		621
Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		113		316
Interest and Other Income [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	33		75
Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1,816		6,013
Revenues		1,844	1,835	6,070	6,050
Segment Reconciling Items [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	10		53
Revenues		15	17	71	74
HCB [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		787		2,421
Revenues		792		2,435
HCB [Member] | Broking [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		54		191
HCB [Member] | Consulting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		576		1,809
HCB [Member] | Outsourced administration [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		70		212
HCB [Member] | Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		73		164
HCB [Member] | Interest and Other Income [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	5		14
HCB [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		773		2,376
Revenues		778	733	2,390	2,408
HCB [Member] | Segment Reconciling Items [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	14		45
CRB [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		606		2,007
Revenues		622		2,036
CRB [Member] | Broking [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		554		1,828
CRB [Member] | Consulting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		37		120
CRB [Member] | Outsourced administration [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		14		53
CRB [Member] | Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1		6
CRB [Member] | Interest and Other Income [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	16		29
CRB [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		606		2,007
Revenues		622	600	2,036	1,916
CRB [Member] | Segment Reconciling Items [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	0		0
IRR [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		308		1,254
Revenues		319		1,281
IRR [Member] | Broking [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		166		781
IRR [Member] | Consulting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		102		326
IRR [Member] | Outsourced administration [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0		0
IRR [Member] | Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		38		142
IRR [Member] | Interest and Other Income [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	11		27
IRR [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		306		1,249
Revenues		317	321	1,276	1,186
IRR [Member] | Segment Reconciling Items [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	2		5
BDA [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		128		373
Revenues		128		373
BDA [Member] | Broking [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		4		12
BDA [Member] | Consulting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0		0
BDA [Member] | Outsourced administration [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		123		356
BDA [Member] | Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0		0
BDA [Member] | Interest and Other Income [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	0		0
BDA [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		127		368
Revenues		127	$ 181	368	$ 540
BDA [Member] | Segment Reconciling Items [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	1		5
Corporate, Non-Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		(3)		11
Revenues		(2)		16
Corporate, Non-Segment [Member] | Broking [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0		0
Corporate, Non-Segment [Member] | Consulting [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		3		9
Corporate, Non-Segment [Member] | Outsourced administration [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		0		0
Corporate, Non-Segment [Member] | Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		1		4
Corporate, Non-Segment [Member] | Interest and Other Income [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[1]	1		5
Corporate, Non-Segment [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax		4		13
Corporate, Non-Segment [Member] | Segment Reconciling Items [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	[2]	$ (7)		$ (2)

[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.