Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Goodwill [Line Items]
Total goodwill	$ 9,036.4	$ 8,666.2
Unites States [Member]
Goodwill [Line Items]
Total goodwill	4,982.4	4,473.9
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	1,997.6	2,213.7
Canada [Member]
Goodwill [Line Items]
Total goodwill	592.2	572.9
Australia [Member]
Goodwill [Line Items]
Total goodwill	467.1	498.1
New Zealand [Member]
Goodwill [Line Items]
Total goodwill	213.0	227.8
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	784.1	679.8
Brokerage [Member]
Goodwill [Line Items]
Total goodwill	8,911.8	8,544.6
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill	4,917.6	4,409.1
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	1,978.1	2,198.7
Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill	592.2	572.9
Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	456.6	487.2
Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	203.5	217.6
Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	763.8	659.1
Risk Management [Member]
Goodwill [Line Items]
Total goodwill	104.3	100.9
Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill	64.8	64.8
Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	19.5	15.0
Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	10.5	10.9
Risk Management [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	9.5	10.2
Corporate [Member]
Goodwill [Line Items]
Total goodwill	20.3	20.7
Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	$ 20.3	$ 20.7