Intangible Assets - Carrying Amount of Goodwill Allocated by Domestic and Foreign Operations (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Total goodwill - net	$ 6,127.0	$ 5,618.5	$ 4,625.6
Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	3,433.0	3,196.9
United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	1,343.4	1,190.7
Canada [Member]
Goodwill [Line Items]
Total goodwill - net	492.9	454.4
Australia [Member]
Goodwill [Line Items]
Total goodwill - net	473.6	427.0
New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	232.6	218.1
Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	151.5	131.4
Brokerage [Member]
Goodwill [Line Items]
Total goodwill - net	6,053.6	5,548.9	4,573.6
Brokerage [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	3,399.8	3,163.8
Brokerage [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	1,328.3	1,177.8
Brokerage [Member] | Canada [Member]
Goodwill [Line Items]
Total goodwill - net	492.9	454.4
Brokerage [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	462.1	416.5
Brokerage [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	221.9	208.0
Brokerage [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	148.6	128.4
Risk Management [Member]
Goodwill [Line Items]
Total goodwill - net	70.5	66.6	49.3
Risk Management [Member] | Unites States [Member]
Goodwill [Line Items]
Total goodwill - net	33.2	33.1
Risk Management [Member] | United Kingdom [Member]
Goodwill [Line Items]
Total goodwill - net	15.1	12.9
Risk Management [Member] | Australia [Member]
Goodwill [Line Items]
Total goodwill - net	11.5	10.5
Risk Management [Member] | New Zealand [Member]
Goodwill [Line Items]
Total goodwill - net	10.7	10.1
Corporate [Member]
Goodwill [Line Items]
Total goodwill - net	2.9	3.0	$ 2.7
Corporate [Member] | Other Foreign [Member]
Goodwill [Line Items]
Total goodwill - net	$ 2.9	$ 3.0