Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Goodwill [Line Items]
Total goodwill	$ 8,727.7	$ 8,666.2
Unites States [Member]
Goodwill [Line Items]
Total goodwill	4,422.0	4,473.9
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,239.8	2,213.7
Canada [Member]
Goodwill [Line Items]
Total goodwill	593.4	572.9
Australia [Member]
Goodwill [Line Items]
Total goodwill	518.5	498.1
New Zealand [Member]
Goodwill [Line Items]
Total goodwill	234.4	227.8
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	719.6	679.8
Brokerage [Member]
Goodwill [Line Items]
Total goodwill	8,605.7	8,544.6
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill	4,357.2	4,409.1
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill	2,225.0	2,198.7
Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill	593.4	572.9
Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	507.2	487.2
Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	223.9	217.6
Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	699.0	659.1
Risk Management [Member]
Goodwill [Line Items]
Total goodwill	101.4	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	14.8	15.0
Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill	11.3	10.9
Risk Management [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill	10.5	10.2
Corporate [Member]
Goodwill [Line Items]
Total goodwill	20.6	20.7
Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill	$ 20.6	$ 20.7