Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Goodwill [Line Items]
Total goodwill	$ 12,193.4	$ 11,475.6
Unites States [Member]
Goodwill [Line Items]
Total goodwill	6,788.9	6,379.3
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,662.8	2,511.9
Canada [Member]
Goodwill [Line Items]
Total goodwill	615.5	623.7
Australia [Member]
Goodwill [Line Items]
Total goodwill	780.5	650.5
New Zealand [Member]
Goodwill [Line Items]
Total goodwill	214.0	213.8
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	1,131.7	1,096.4
Brokerage [Member]
Goodwill [Line Items]
Total goodwill	11,829.3	11,217.8
Risk Management [Member]
Goodwill [Line Items]
Total goodwill	345.2	238.8
Corporate [Member]
Goodwill [Line Items]
Total goodwill	18.9	19.0
Operating Segments [Member] \| Brokerage [Member]
Goodwill [Line Items]
Total goodwill	11,829.3	11,217.8
Operating Segments [Member] \| Brokerage [Member] \| Unites States [Member]
Goodwill [Line Items]
Total goodwill	6,714.1	6,304.5
Operating Segments [Member] \| Brokerage [Member] \| United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,643.3	2,493.4
Operating Segments [Member] \| Brokerage [Member] \| Canada [Member]
Goodwill [Line Items]
Total goodwill	615.5	623.7
Operating Segments [Member] \| Brokerage [Member] \| Australia [Member]
Goodwill [Line Items]
Total goodwill	539.1	514.6
Operating Segments [Member] \| Brokerage [Member] \| New Zealand [Member]
Goodwill [Line Items]
Total goodwill	204.5	204.2
Operating Segments [Member] \| Brokerage [Member] \| Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	1,112.8	1,077.4
Operating Segments [Member] \| Risk Management [Member]
Goodwill [Line Items]
Total goodwill	345.2	238.8
Operating Segments [Member] \| Risk Management [Member] \| Unites States [Member]
Goodwill [Line Items]
Total goodwill	74.8	74.8
Operating Segments [Member] \| Risk Management [Member] \| United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	19.5	18.5
Operating Segments [Member] \| Risk Management [Member] \| Australia [Member]
Goodwill [Line Items]
Total goodwill	241.4	135.9
Operating Segments [Member] \| Risk Management [Member] \| New Zealand [Member]
Goodwill [Line Items]
Total goodwill	9.5	9.6
Operating Segments [Member] \| Corporate [Member]
Goodwill [Line Items]
Total goodwill	18.9	19.0
Operating Segments [Member] \| Corporate [Member] \| Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	$ 18.9	$ 19.0