Segment Information - Summary of Geographical Location and Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 1,933.5	$ 1,584.0	$ 4,098.5	$ 3,450.9
Unites States [Member]
Segment Reporting Information [Line Items]
Total revenues	1,305.5	1,086.1	2,921.7	2,486.5
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total revenues	337.9	269.1	641.4	535.0
Australia [Member]
Segment Reporting Information [Line Items]
Total revenues	103.7	81.6	186.7	146.3
Canada [Member]
Segment Reporting Information [Line Items]
Total revenues	81.3	59.7	150.2	116.9
New Zealand [Member]
Segment Reporting Information [Line Items]
Total revenues	49.4	41.2	85.6	72.0
Other Foreign [Member]
Segment Reporting Information [Line Items]
Total revenues	55.7	46.3	112.9	94.2
Brokerage [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1,390.2	1,201.1	3,000.4	2,636.7
Brokerage [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	812.4	740.8	1,919.0	1,746.7
Brokerage [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	326.1	260.9	618.5	515.4
Brokerage [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	70.7	56.9	124.7	100.0
Brokerage [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	79.9	58.2	147.4	114.4
Brokerage [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	45.4	38.0	77.9	66.0
Brokerage [Member] \| Other Foreign [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	55.7	46.3	112.9	94.2
Risk Management [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	281.7	223.2	534.4	472.7
Risk Management [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	231.5	185.6	439.0	398.3
Risk Management [Member] \| United Kingdom [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	11.8	8.2	22.9	19.6
Risk Management [Member] \| Australia [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	33.0	24.7	62.0	46.3
Risk Management [Member] \| Canada [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	1.4	1.5	2.8	2.5
Risk Management [Member] \| New Zealand [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	4.0	3.2	7.7	6.0
Corporate [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	261.6	159.7	563.7	341.5
Corporate [Member] \| Unites States [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 261.6	$ 159.7	$ 563.7	$ 341.5