Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Goodwill [Line Items]
Total goodwill - net	$ 11,475.6	$ 9,489.4	$ 8,666.2
Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	6,379.3	5,140.5
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	2,511.9	2,197.9
Canada [Member]
Goodwill [Line Items]
Total goodwill - net	623.7	569.7
Australia [Member]
Goodwill [Line Items]
Total goodwill - net	650.5	477.8
New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	213.8	213.3
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	1,096.4	890.2
Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	11,217.8	9,358.1	8,544.6
Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	238.8	112.2	100.9
Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	19.0	19.1	20.7
Operating Segments [Member] | Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	11,217.8	9,358.1	8,544.6
Operating Segments [Member] | Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	6,304.5	5,065.7
Operating Segments [Member] | Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	2,493.4	2,180.2
Operating Segments [Member] | Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill - net	623.7	569.7
Operating Segments [Member] | Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	514.6	467.6
Operating Segments [Member] | Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	204.2	203.8
Operating Segments [Member] | Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	1,077.4	871.1
Operating Segments [Member] | Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	238.8	112.2	100.9
Operating Segments [Member] | Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	74.8	74.8
Operating Segments [Member] | Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	18.5	17.7
Operating Segments [Member] | Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	135.9	10.2
Operating Segments [Member] | Risk Management [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	9.6	9.5
Operating Segments [Member] | Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	19.0	19.1	$ 20.7
Operating Segments [Member] | Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	$ 19.0	$ 19.1