Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Goodwill [Line Items]
Total goodwill - net	$ 4,625.6	$ 4,164.8	[1]	$ 3,753.2
Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	2,744.9	2,306.7
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	762.9	745.7
Canada [Member]
Goodwill [Line Items]
Total goodwill - net	378.6	374.0
Australia [Member]
Goodwill [Line Items]
Total goodwill - net	406.6	416.6
New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	219.8	218.9
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	112.8	102.9
Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	4,573.6	4,119.2		3,722.3
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	2,715.3	2,280.9
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	753.7	738.5
Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill - net	378.6	374.0
Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	406.3	416.6
Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	209.6	209.3
Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	110.1	99.9
Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	49.3	42.6		28.1
Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	29.6	25.8
Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	9.2	7.2
Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	0.3
Risk Management [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	10.2	9.6
Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	2.7	3.0		$ 2.8
Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	$ 2.7	$ 3.0

[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.