Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Line Items]
Total goodwill - net	$ 12,270.2	$ 11,475.6	$ 9,489.4
Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	7,040.4	6,379.3
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	2,617.1	2,511.9
Canada [Member]
Goodwill [Line Items]
Total goodwill - net	586.9	623.7
Australia [Member]
Goodwill [Line Items]
Total goodwill - net	728.4	650.5
New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	191.7	213.8
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	1,105.7	1,096.4
Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	11,923.4	11,217.8	9,358.1
Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	328.3	238.8	112.2
Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	18.5	19.0	19.1
Operating Segments [Member] \| Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	11,923.4	11,217.8	9,358.1
Operating Segments [Member] \| Brokerage [Member] \| Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	6,965.6	6,304.5
Operating Segments [Member] \| Brokerage [Member] \| United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	2,591.4	2,493.4
Operating Segments [Member] \| Brokerage [Member] \| Canada [Member]
Goodwill [Line Items]
Total goodwill - net	586.9	623.7
Operating Segments [Member] \| Brokerage [Member] \| Australia [Member]
Goodwill [Line Items]
Total goodwill - net	509.1	514.6
Operating Segments [Member] \| Brokerage [Member] \| New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	183.2	204.2
Operating Segments [Member] \| Brokerage [Member] \| Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	1,087.2	1,077.4
Operating Segments [Member] \| Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	328.3	238.8	112.2
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	25.7	18.5
Operating Segments [Member] \| Risk Management [Member] \| Australia [Member]
Goodwill [Line Items]
Total goodwill - net	219.3	135.9
Operating Segments [Member] \| Risk Management [Member] \| New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	8.5	9.6
Operating Segments [Member] \| Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	18.5	19.0	$ 19.1
Operating Segments [Member] \| Corporate [Member] \| Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	$ 18.5	$ 19.0