Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Goodwill [Line Items]
Total goodwill	$ 10,469.4	$ 9,489.4
Unites States [Member]
Goodwill [Line Items]
Total goodwill	5,823.1	5,140.5
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,340.7	2,197.9
Canada [Member]
Goodwill [Line Items]
Total goodwill	591.1	569.7
Australia [Member]
Goodwill [Line Items]
Total goodwill	489.3	477.8
New Zealand [Member]
Goodwill [Line Items]
Total goodwill	200.5	213.3
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	1,024.7	890.2
Brokerage [Member]
Goodwill [Line Items]
Total goodwill	10,338.9	9,358.1
Risk Management [Member]
Goodwill [Line Items]
Total goodwill	111.5	112.2
Corporate [Member]
Goodwill [Line Items]
Total goodwill	19.0	19.1
Operating Segments [Member] \| Brokerage [Member]
Goodwill [Line Items]
Total goodwill	10,338.9	9,358.1
Operating Segments [Member] \| Brokerage [Member] \| Unites States [Member]
Goodwill [Line Items]
Total goodwill	5,748.3	5,065.7
Operating Segments [Member] \| Brokerage [Member] \| United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,322.8	2,180.2
Operating Segments [Member] \| Brokerage [Member] \| Canada [Member]
Goodwill [Line Items]
Total goodwill	591.1	569.7
Operating Segments [Member] \| Brokerage [Member] \| Australia [Member]
Goodwill [Line Items]
Total goodwill	479.5	467.6
Operating Segments [Member] \| Brokerage [Member] \| New Zealand [Member]
Goodwill [Line Items]
Total goodwill	191.5	203.8
Operating Segments [Member] \| Brokerage [Member] \| Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	1,005.7	871.1
Operating Segments [Member] \| Risk Management [Member]
Goodwill [Line Items]
Total goodwill	111.5	112.2
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	17.9	17.7
Operating Segments [Member] \| Risk Management [Member] \| Australia [Member]
Goodwill [Line Items]
Total goodwill	9.8	10.2
Operating Segments [Member] \| Risk Management [Member] \| New Zealand [Member]
Goodwill [Line Items]
Total goodwill	9.0	9.5
Operating Segments [Member] \| Corporate [Member]
Goodwill [Line Items]
Total goodwill	19.0	19.1
Operating Segments [Member] \| Corporate [Member] \| Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	$ 19.0	$ 19.1