Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 2,138.2	$ 1,849.1	$ 6,236.7	$ 5,300.0
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,592.1	1,380.7	4,513.8	3,867.2
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	288.6	240.6	930.0	775.6
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	92.0	82.8	278.7	229.1
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	69.7	58.5	219.9	175.4
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	44.4	40.0	130.0	112.0
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	51.4	46.5	164.3	140.7
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,499.7	1,294.6	4,500.1	3,931.3
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,000.3	866.6	2,919.3	2,613.3
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	276.5	230.8	895.0	746.2
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	62.4	56.6	187.1	156.6
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	68.1	57.1	215.5	171.5
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	41.0	37.0	118.9	103.0
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	51.4	46.5	164.3	140.7
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	280.6	244.3	815.0	717.0
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	233.9	203.9	672.9	602.2
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	12.1	9.8	35.0	29.4
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	29.6	26.2	91.6	72.5
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.6	1.4	4.4	3.9
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	3.4	3.0	11.1	9.0
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	357.9	310.2	921.6	651.7
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 357.9	$ 310.2	$ 921.6	$ 651.7